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+Internet Engineering Task Force (IETF)                    R. Rahman, Ed.
+Request for Comments: 9127                                              
+Category: Standards Track                                  L. Zheng, Ed.
+ISSN: 2070-1721                                      Huawei Technologies
+                                                    M. Jethanandani, Ed.
+                                                     Xoriant Corporation
+                                                           S. Pallagatti
+                                                                  VMware
+                                                               G. Mirsky
+                                                                Ericsson
+                                                            October 2021
+
+
+      YANG Data Model for Bidirectional Forwarding Detection (BFD)
+
+Abstract
+
+   This document defines a YANG data model that can be used to configure
+   and manage Bidirectional Forwarding Detection (BFD).
+
+   The YANG modules in this document conform to the Network Management
+   Datastore Architecture (NMDA) (RFC 8342).
+
+Status of This Memo
+
+   This is an Internet Standards Track document.
+
+   This document is a product of the Internet Engineering Task Force
+   (IETF).  It represents the consensus of the IETF community.  It has
+   received public review and has been approved for publication by the
+   Internet Engineering Steering Group (IESG).  Further information on
+   Internet Standards is available in Section 2 of RFC 7841.
+
+   Information about the current status of this document, any errata,
+   and how to provide feedback on it may be obtained at
+   https://www.rfc-editor.org/info/rfc9127.
+
+Copyright Notice
+
+   Copyright (c) 2021 IETF Trust and the persons identified as the
+   document authors.  All rights reserved.
+
+   This document is subject to BCP 78 and the IETF Trust's Legal
+   Provisions Relating to IETF Documents
+   (https://trustee.ietf.org/license-info) in effect on the date of
+   publication of this document.  Please review these documents
+   carefully, as they describe your rights and restrictions with respect
+   to this document.  Code Components extracted from this document must
+   include Simplified BSD License text as described in Section 4.e of
+   the Trust Legal Provisions and are provided without warranty as
+   described in the Simplified BSD License.
+
+Table of Contents
+
+   1.  Introduction
+     1.1.  Tree Diagrams
+   2.  Design of the Data Model
+     2.1.  Design of the Configuration Model
+       2.1.1.  Common BFD Configuration Parameters
+       2.1.2.  Single-Hop IP
+       2.1.3.  Multihop IP
+       2.1.4.  MPLS Label Switched Paths
+       2.1.5.  Link Aggregation Groups
+     2.2.  Design of the Operational State Model
+     2.3.  Notifications
+     2.4.  RPC Operations
+     2.5.  BFD Top-Level Hierarchy
+     2.6.  BFD IP Single-Hop Hierarchy
+     2.7.  BFD IP Multihop Hierarchy
+     2.8.  BFD-over-LAG Hierarchy
+     2.9.  BFD-over-MPLS-LSPs Hierarchy
+     2.10. Interaction with Other YANG Modules
+       2.10.1.  "ietf-interfaces" Module
+       2.10.2.  "ietf-ip" Module
+       2.10.3.  "ietf-mpls" Module
+     2.11. IANA BFD YANG Module
+     2.12. BFD Types YANG Module
+     2.13. BFD Top-Level YANG Module
+     2.14. BFD IP Single-Hop YANG Module
+     2.15. BFD IP Multihop YANG Module
+     2.16. BFD-over-LAG YANG Module
+     2.17. BFD-over-MPLS YANG Module
+   3.  Data Model Examples
+     3.1.  IP Single-Hop
+     3.2.  IP Multihop
+     3.3.  LAG
+     3.4.  MPLS
+   4.  Security Considerations
+   5.  IANA Considerations
+     5.1.  IANA-Maintained "iana-bfd-types" Module
+   6.  References
+     6.1.  Normative References
+     6.2.  Informative References
+   Appendix A.  Echo Function Configuration Example
+     A.1.  Example YANG Module for BFD Echo Function Configuration
+   Acknowledgments
+   Authors' Addresses
+
+1.  Introduction
+
+   This document defines a YANG data model that can be used to configure
+   and manage Bidirectional Forwarding Detection (BFD) [RFC5880].  BFD
+   is a network protocol that is used for liveness detection of
+   arbitrary paths between systems.  Some examples of different types of
+   paths over which we have BFD are as follows:
+
+   1.  Two systems directly connected via IP.  This is known as BFD over
+       single-hop IP, a.k.a. BFD for IPv4 and IPv6 [RFC5881].
+
+   2.  Two systems connected via multiple hops as described in
+       "Bidirectional Forwarding Detection (BFD) for Multihop Paths"
+       [RFC5883].
+
+   3.  Two systems connected via MPLS Label Switched Paths (LSPs) as
+       described in "Bidirectional Forwarding Detection (BFD) for MPLS
+       Label Switched Paths (LSPs)" [RFC5884].
+
+   4.  Two systems connected via a Link Aggregation Group (LAG)
+       interface as described in "Bidirectional Forwarding Detection
+       (BFD) on Link Aggregation Group (LAG) Interfaces" [RFC7130].
+
+   5.  Two systems connected via pseudowires (PWs).  This is known as
+       Virtual Circuit Connectivity Verification (VCCV), as described in
+       "Bidirectional Forwarding Detection (BFD) for the Pseudowire
+       Virtual Circuit Connectivity Verification (VCCV)" [RFC5885].
+       This scenario is not addressed in this document.
+
+   BFD typically does not operate on its own.  Various control
+   protocols, also known as BFD clients, use the services provided by
+   BFD for their own operation, as described in "Generic Application of
+   Bidirectional Forwarding Detection (BFD)" [RFC5882].  The obvious
+   candidates that use BFD are those that do not have "hellos" to detect
+   failures, e.g., static routes, and routing protocols whose "hellos"
+   do not support sub-second failure detection, e.g., OSPF and IS-IS.
+
+   The YANG modules in this document conform to the Network Management
+   Datastore Architecture (NMDA) [RFC8342].  This means that the data
+   models do not have separate top-level or sibling containers for
+   configuration data and operational state data.
+
+1.1.  Tree Diagrams
+
+   This document uses the graphical representation of data models, as
+   defined in [RFC8340].
+
+2.  Design of the Data Model
+
+   Since BFD is used for liveness detection of various forwarding paths,
+   there is no uniform key to identify a BFD session, and so the BFD
+   data model is split into multiple YANG modules where each module
+   corresponds to one type of forwarding path.  For example, BFD for IP
+   single-hop is in one YANG module, and BFD for MPLS is in another YANG
+   module.  The main difference between these modules is how a BFD
+   session is uniquely identified, i.e., the key for the list containing
+   the BFD sessions for that forwarding path.  To avoid duplication of
+   BFD definitions, we have common types and groupings that are used by
+   all the modules.
+
+   A new control-plane protocol, "bfdv1", is defined, and a "bfd"
+   container is created under "control-plane-protocol" as specified in
+   "A YANG Data Model for Routing Management (NMDA Version)" [RFC8349].
+   This new "bfd" container is augmented by the following YANG modules
+   for their respective specific information:
+
+   1.  The "ietf-bfd-ip-sh" module (Section 2.14) augments "/routing/
+       control-plane-protocols/control-plane-protocol/bfd/" with the
+       "ip-sh" container for BFD sessions over IP single-hop.
+
+   2.  The "ietf-bfd-ip-mh" module (Section 2.15) augments "/routing/
+       control-plane-protocols/control-plane-protocol/bfd/" with the
+       "ip-mh" container for BFD sessions over IP multihop.
+
+   3.  The "ietf-bfd-lag" module (Section 2.16) augments "/routing/
+       control-plane-protocols/control-plane-protocol/bfd/" with the
+       "lag" container for BFD sessions over a LAG.
+
+   4.  The "ietf-bfd-mpls" module (Section 2.17) augments "/routing/
+       control-plane-protocols/control-plane-protocol/bfd/" with the
+       "mpls" container for BFD-over-MPLS LSPs.
+
+   BFD can operate in the following contexts:
+
+   1.  At the network device level.
+
+   2.  In logical network elements (LNEs) as described in "YANG Model
+       for Logical Network Elements" [RFC8530].
+
+   3.  In network instances as described in "YANG Data Model for Network
+       Instances" [RFC8529].
+
+   When used at the network device level, the BFD YANG data model is
+   used "as is".  When the BFD YANG data model is used in an LNE or
+   network instance, the BFD YANG data model augments the mounted
+   routing model for the LNE or network instance.
+
+2.1.  Design of the Configuration Model
+
+   The configuration model consists mainly of the parameters specified
+   in BFD [RFC5880] -- for example, desired minimum transmit interval,
+   required minimum receive interval, and detection multiplier.
+
+   BFD clients are applications that use BFD for fast detection of
+   failures.  Some implementations have BFD session configuration under
+   the BFD clients -- for example, BFD session configuration under
+   routing applications such as OSPF, IS-IS, or BGP.  Other
+   implementations have BFD session configuration centralized under BFD,
+   i.e., outside the multiple BFD clients.
+
+   The main BFD parameters of interest to a BFD client are those related
+   to the multiplier and interval(s), since those parameters impact the
+   convergence time of the BFD clients when a failure occurs.  Other
+   parameters, such as BFD authentication, are not specific to the
+   requirements of the BFD client.  Configuration of BFD for all clients
+   should be centralized.  However, this is a problem for BFD clients
+   that auto-discover their peers.  For example, IGPs do not have the
+   peer address configured; instead, the IGP is enabled on an interface,
+   and the IGP peers are auto-discovered.  So, for an operator to
+   configure BFD to an IGP peer, the operator would first have to
+   determine the peer addresses.  And when a new peer is discovered, BFD
+   configuration would need to be added.  To avoid this issue, we define
+   the grouping "client-cfg-parms" in Section 2.12 for BFD clients to
+   configure BFD: this allows BFD clients, such as the IGPs, to have
+   configuration (multiplier and intervals) for the BFD sessions they
+   need.  For example, when a new IGP peer is discovered, the IGP would
+   create a BFD session to the newly discovered peer; similarly, when an
+   IGP peer goes away, the IGP would remove the BFD session to that
+   peer.  The mechanism for how the BFD sessions are created and removed
+   by the BFD clients is outside the scope of this document, but this
+   would typically be done by using an API implemented by the BFD module
+   on the system.  In the case of BFD clients that create BFD sessions
+   via their own configuration, authentication parameters (if required)
+   are still specified in BFD.
+
+2.1.1.  Common BFD Configuration Parameters
+
+   The basic BFD configuration parameters are as follows:
+
+   local-multiplier
+      This is the detection time multiplier as defined in BFD [RFC5880].
+
+   desired-min-tx-interval
+      This is the Desired Min TX Interval as defined in BFD [RFC5880].
+
+   required-min-rx-interval
+      This is the Required Min RX Interval as defined in BFD [RFC5880].
+
+   Although BFD [RFC5880] allows for different values for transmit and
+   receive intervals, some implementations allow users to specify just
+   one interval that is used for both transmit and receive intervals, or
+   separate values for transmit and receive intervals.  The BFD YANG
+   data model supports this: there is a choice between "min-interval",
+   used for both transmit and receive intervals, and "desired-min-tx-
+   interval" and "required-min-rx-interval".  This is supported via the
+   "base-cfg-parms" grouping (Section 2.12), which is used by the YANG
+   modules for the various forwarding paths.
+
+   For BFD authentication, we have the following:
+
+   key-chain
+      This is a reference to "key-chain" as defined in "YANG Data Model
+      for Key Chains" [RFC8177].  The keys, cryptographic algorithms,
+      key lifetime, etc. are all defined in the "key-chain" model.
+
+   meticulous
+      This enables a meticulous mode as per BFD [RFC5880].
+
+2.1.2.  Single-Hop IP
+
+   For single-hop IP, there is an augment of the "bfd" data node, as
+   described in Section 2.  The "ip-sh" node contains a list of IP
+   single-hop sessions where each session is uniquely identified by the
+   interface and destination address pair.  We use the configuration
+   parameters defined in Section 2.1.1.  The "ip-sh" node also contains
+   a list of interfaces and is used to specify authentication parameters
+   for BFD sessions that are created by BFD clients.  See Section 2.1.
+
+   [RFC5880] and [RFC5881] do not specify whether the Echo function
+   operates continuously or on demand.  Therefore, the mechanism used to
+   start and stop the Echo function is implementation specific and
+   should be done by augmentation:
+
+   1.  Configuration.  This is suitable for an Echo function that
+       operates continuously.  An example is provided in Appendix A.
+
+   2.  RPC.  This is suitable for an Echo function that operates on
+       demand.
+
+2.1.3.  Multihop IP
+
+   For multihop IP, there is an augment of the "bfd" data node, as
+   described in Section 2.
+
+   Because of multiple paths, there could be multiple multihop IP
+   sessions between a source and a destination address.  We identify
+   this set of sessions as a "session-group".  The key for each
+   "session-group" consists of the following:
+
+   Source address
+      Address belonging to the local system as per "Bidirectional
+      Forwarding Detection (BFD) for Multihop Paths" [RFC5883].
+
+   Destination address
+      Address belonging to the remote system as per [RFC5883].
+
+   We use the configuration parameters defined in Section 2.1.1.
+
+   This document also provides the following parameters:
+
+   tx-ttl
+      TTL of outgoing BFD control packets.
+
+   rx-ttl
+      Minimum TTL of incoming BFD control packets.
+
+2.1.4.  MPLS Label Switched Paths
+
+   Here, we address MPLS LSPs whose Forwarding Equivalence Class (FEC)
+   [RFC3031] is an IP address.  The "bfd" node (Section 2) is augmented
+   with "mpls", which contains a list of sessions uniquely identified by
+   an IP prefix.  Because of multiple paths, there could be multiple
+   MPLS sessions to an MPLS FEC.  We identify this set of sessions as a
+   "session-group".
+
+   Since these LSPs are unidirectional, there is no LSP configuration on
+   the egress node.
+
+   The BFD parameters for the egress node are added under "mpls".
+
+2.1.5.  Link Aggregation Groups
+
+   Per "Bidirectional Forwarding Detection (BFD) on Link Aggregation
+   Group (LAG) Interfaces" [RFC7130], configuring BFD on a LAG consists
+   of having micro-BFD sessions on each LAG member link.  Since the BFD
+   parameters are an attribute of the LAG, they should be under the LAG.
+   However, there is no LAG YANG data model that we can augment.  So, a
+   "lag" data node is added to the "bfd" node; see Section 2.  The
+   configuration is per LAG: we have a list of LAGs.  The destination IP
+   address of the micro-BFD sessions is configured per LAG and per
+   address family (IPv4 and IPv6).
+
+2.2.  Design of the Operational State Model
+
+   The operational state model contains both the overall statistics for
+   the BFD sessions running on the device and the per-session
+   operational information.
+
+   The overall statistics for the BFD sessions consist of the number of
+   BFD sessions, the number of BFD sessions that are up, etc.  This
+   information is available globally (i.e., for all BFD sessions) under
+   the "bfd" node (Section 2) and also per type of forwarding path.
+
+   For each BFD session, three main categories of operational state data
+   are shown.
+
+   1.  The first category includes fundamental information regarding a
+       BFD session, such as the local discriminator, the remote
+       discriminator, and the ability to support Demand mode.
+
+   2.  The second category includes BFD "session-running" information,
+       e.g., the remote BFD state and the diagnostic code received.
+       Another example is the actual transmit interval between the
+       control packets, which may be different from the configured
+       desired minimum transmit interval.  Similar examples include the
+       actual receive interval between the control packets and the
+       actual transmit interval between the Echo packets.
+
+   3.  The third category contains the detailed statistics for the
+       session, e.g., when the session transitioned up/down and how long
+       it has been in that state.
+
+   For some path types, there may be more than one session on the
+   virtual path to the destination.  For example, with IP multihop and
+   MPLS LSPs, there could be multiple BFD sessions from the source to
+   the same destination to test the various paths (ECMP) to the
+   destination.  This is represented by having multiple "sessions" under
+   each "session-group".
+
+2.3.  Notifications
+
+   This YANG data model defines notifications to inform end users of
+   important events detected during the protocol operation.  The local
+   discriminator identifies the corresponding BFD session on the local
+   system, and the remote discriminator identifies the BFD session on
+   the remote system.  Notifications also give more important details
+   about BFD sessions, e.g., new state, time in previous state, network
+   instance, and the reason that the BFD session state changed.  The
+   notifications are defined for each type of forwarding path but use
+   groupings for common information.
+
+2.4.  RPC Operations
+
+   None.
+
+2.5.  BFD Top-Level Hierarchy
+
+   At the "bfd" node under "control-plane-protocol", there is no
+   configuration data -- only operational state data.  The operational
+   state data consists of overall BFD session statistics, i.e., for BFD
+   on all types of forwarding paths.
+
+   module: ietf-bfd
+     augment /rt:routing/rt:control-plane-protocols
+               /rt:control-plane-protocol:
+       +--rw bfd
+          +--ro summary
+             +--ro number-of-sessions?              yang:gauge32
+             +--ro number-of-sessions-up?           yang:gauge32
+             +--ro number-of-sessions-down?         yang:gauge32
+             +--ro number-of-sessions-admin-down?   yang:gauge32
+
+2.6.  BFD IP Single-Hop Hierarchy
+
+   An "ip-sh" node is added under the "bfd" node in "control-plane-
+   protocol".  The configuration data and operational state data for
+   each BFD IP single-hop session are under this "ip-sh" node.
+
+   module: ietf-bfd-ip-sh
+     augment /rt:routing/rt:control-plane-protocols
+               /rt:control-plane-protocol/bfd:bfd:
+       +--rw ip-sh
+          +--ro summary
+          |  +--ro number-of-sessions?              yang:gauge32
+          |  +--ro number-of-sessions-up?           yang:gauge32
+          |  +--ro number-of-sessions-down?         yang:gauge32
+          |  +--ro number-of-sessions-admin-down?   yang:gauge32
+          +--rw sessions
+          |  +--rw session* [interface dest-addr]
+          |     +--rw interface                         if:interface-ref
+          |     +--rw dest-addr                         inet:ip-address
+          |     +--rw source-addr?                      inet:ip-address
+          |     +--rw local-multiplier?                 multiplier
+          |     +--rw (interval-config-type)?
+          |     |  +--:(tx-rx-intervals)
+          |     |  |  +--rw desired-min-tx-interval?    uint32
+          |     |  |  +--rw required-min-rx-interval?   uint32
+          |     |  +--:(single-interval) {single-minimum-interval}?
+          |     |     +--rw min-interval?               uint32
+          |     +--rw demand-enabled?                   boolean
+          |     |       {demand-mode}?
+          |     +--rw admin-down?                       boolean
+          |     +--rw authentication! {authentication}?
+          |     |  +--rw key-chain?    key-chain:key-chain-ref
+          |     |  +--rw meticulous?   boolean
+          |     +--ro path-type?                        identityref
+          |     +--ro ip-encapsulation?                 boolean
+          |     +--ro local-discriminator?              discriminator
+          |     +--ro remote-discriminator?             discriminator
+          |     +--ro remote-multiplier?                multiplier
+          |     +--ro demand-capability?                boolean
+          |     |       {demand-mode}?
+          |     +--ro source-port?                      inet:port-number
+          |     +--ro dest-port?                        inet:port-number
+          |     +--ro session-running
+          |     |  +--ro session-index?                uint32
+          |     |  +--ro local-state?                  state
+          |     |  +--ro remote-state?                 state
+          |     |  +--ro local-diagnostic?
+          |     |  |       iana-bfd-types:diagnostic
+          |     |  +--ro remote-diagnostic?
+          |     |  |       iana-bfd-types:diagnostic
+          |     |  +--ro remote-authenticated?         boolean
+          |     |  +--ro remote-authentication-type?
+          |     |  |       iana-bfd-types:auth-type {authentication}?
+          |     |  +--ro detection-mode?               enumeration
+          |     |  +--ro negotiated-tx-interval?       uint32
+          |     |  +--ro negotiated-rx-interval?       uint32
+          |     |  +--ro detection-time?               uint32
+          |     |  +--ro echo-tx-interval-in-use?      uint32
+          |     |          {echo-mode}?
+          |     +--ro session-statistics
+          |        +--ro create-time?
+          |        |       yang:date-and-time
+          |        +--ro last-down-time?
+          |        |       yang:date-and-time
+          |        +--ro last-up-time?
+          |        |       yang:date-and-time
+          |        +--ro down-count?                     yang:counter32
+          |        +--ro admin-down-count?               yang:counter32
+          |        +--ro receive-packet-count?           yang:counter64
+          |        +--ro send-packet-count?              yang:counter64
+          |        +--ro receive-invalid-packet-count?   yang:counter64
+          |        +--ro send-failed-packet-count?       yang:counter64
+          +--rw interfaces* [interface]
+             +--rw interface         if:interface-ref
+             +--rw authentication! {authentication}?
+                +--rw key-chain?    key-chain:key-chain-ref
+                +--rw meticulous?   boolean
+
+     notifications:
+       +---n singlehop-notification
+          +--ro local-discr?                 discriminator
+          +--ro remote-discr?                discriminator
+          +--ro new-state?                   state
+          +--ro state-change-reason?         iana-bfd-types:diagnostic
+          +--ro time-of-last-state-change?   yang:date-and-time
+          +--ro dest-addr?                   inet:ip-address
+          +--ro source-addr?                 inet:ip-address
+          +--ro session-index?               uint32
+          +--ro path-type?                   identityref
+          +--ro interface?                   if:interface-ref
+          +--ro echo-enabled?                boolean
+
+2.7.  BFD IP Multihop Hierarchy
+
+   An "ip-mh" node is added under the "bfd" node in "control-plane-
+   protocol".  The configuration data and operational state data for
+   each BFD IP multihop session are under this "ip-mh" node.  In the
+   operational state model, we support multiple BFD multihop sessions
+   per remote address (ECMP); the local discriminator is used as the
+   key.
+
+   module: ietf-bfd-ip-mh
+     augment /rt:routing/rt:control-plane-protocols
+               /rt:control-plane-protocol/bfd:bfd:
+       +--rw ip-mh
+          +--ro summary
+          |  +--ro number-of-sessions?              yang:gauge32
+          |  +--ro number-of-sessions-up?           yang:gauge32
+          |  +--ro number-of-sessions-down?         yang:gauge32
+          |  +--ro number-of-sessions-admin-down?   yang:gauge32
+          +--rw session-groups
+             +--rw session-group* [source-addr dest-addr]
+                +--rw source-addr                       inet:ip-address
+                +--rw dest-addr                         inet:ip-address
+                +--rw local-multiplier?                 multiplier
+                +--rw (interval-config-type)?
+                |  +--:(tx-rx-intervals)
+                |  |  +--rw desired-min-tx-interval?    uint32
+                |  |  +--rw required-min-rx-interval?   uint32
+                |  +--:(single-interval) {single-minimum-interval}?
+                |     +--rw min-interval?               uint32
+                +--rw demand-enabled?                   boolean
+                |       {demand-mode}?
+                +--rw admin-down?                       boolean
+                +--rw authentication! {authentication}?
+                |  +--rw key-chain?    key-chain:key-chain-ref
+                |  +--rw meticulous?   boolean
+                +--rw tx-ttl?                           bfd-types:hops
+                +--rw rx-ttl                            bfd-types:hops
+                +--ro sessions* []
+                   +--ro path-type?              identityref
+                   +--ro ip-encapsulation?       boolean
+                   +--ro local-discriminator?    discriminator
+                   +--ro remote-discriminator?   discriminator
+                   +--ro remote-multiplier?      multiplier
+                   +--ro demand-capability?      boolean {demand-mode}?
+                   +--ro source-port?            inet:port-number
+                   +--ro dest-port?              inet:port-number
+                   +--ro session-running
+                   |  +--ro session-index?                uint32
+                   |  +--ro local-state?                  state
+                   |  +--ro remote-state?                 state
+                   |  +--ro local-diagnostic?
+                   |  |       iana-bfd-types:diagnostic
+                   |  +--ro remote-diagnostic?
+                   |  |       iana-bfd-types:diagnostic
+                   |  +--ro remote-authenticated?         boolean
+                   |  +--ro remote-authentication-type?
+                   |  |       iana-bfd-types:auth-type {authentication}?
+                   |  +--ro detection-mode?               enumeration
+                   |  +--ro negotiated-tx-interval?       uint32
+                   |  +--ro negotiated-rx-interval?       uint32
+                   |  +--ro detection-time?               uint32
+                   |  +--ro echo-tx-interval-in-use?      uint32
+                   |          {echo-mode}?
+                   +--ro session-statistics
+                      +--ro create-time?
+                      |       yang:date-and-time
+                      +--ro last-down-time?
+                      |       yang:date-and-time
+                      +--ro last-up-time?
+                      |       yang:date-and-time
+                      +--ro down-count?
+                      |       yang:counter32
+                      +--ro admin-down-count?
+                      |       yang:counter32
+                      +--ro receive-packet-count?
+                      |       yang:counter64
+                      +--ro send-packet-count?
+                      |       yang:counter64
+                      +--ro receive-invalid-packet-count?
+                      |       yang:counter64
+                      +--ro send-failed-packet-count?
+                              yang:counter64
+
+     notifications:
+       +---n multihop-notification
+          +--ro local-discr?                 discriminator
+          +--ro remote-discr?                discriminator
+          +--ro new-state?                   state
+          +--ro state-change-reason?         iana-bfd-types:diagnostic
+          +--ro time-of-last-state-change?   yang:date-and-time
+          +--ro dest-addr?                   inet:ip-address
+          +--ro source-addr?                 inet:ip-address
+          +--ro session-index?               uint32
+          +--ro path-type?                   identityref
+
+2.8.  BFD-over-LAG Hierarchy
+
+   A "lag" node is added under the "bfd" node in "control-plane-
+   protocol".  The configuration data and operational state data for
+   each BFD LAG session are under this "lag" node.
+
+   module: ietf-bfd-lag
+     augment /rt:routing/rt:control-plane-protocols
+               /rt:control-plane-protocol/bfd:bfd:
+       +--rw lag
+          +--rw micro-bfd-ipv4-session-statistics
+          |  +--ro summary
+          |     +--ro number-of-sessions?              yang:gauge32
+          |     +--ro number-of-sessions-up?           yang:gauge32
+          |     +--ro number-of-sessions-down?         yang:gauge32
+          |     +--ro number-of-sessions-admin-down?   yang:gauge32
+          +--rw micro-bfd-ipv6-session-statistics
+          |  +--ro summary
+          |     +--ro number-of-sessions?              yang:gauge32
+          |     +--ro number-of-sessions-up?           yang:gauge32
+          |     +--ro number-of-sessions-down?         yang:gauge32
+          |     +--ro number-of-sessions-admin-down?   yang:gauge32
+          +--rw sessions
+             +--rw session* [lag-name]
+                +--rw lag-name                          if:interface-ref
+                +--rw ipv4-dest-addr?
+                |       inet:ipv4-address
+                +--rw ipv6-dest-addr?
+                |       inet:ipv6-address
+                +--rw local-multiplier?                 multiplier
+                +--rw (interval-config-type)?
+                |  +--:(tx-rx-intervals)
+                |  |  +--rw desired-min-tx-interval?    uint32
+                |  |  +--rw required-min-rx-interval?   uint32
+                |  +--:(single-interval) {single-minimum-interval}?
+                |     +--rw min-interval?               uint32
+                +--rw demand-enabled?                   boolean
+                |       {demand-mode}?
+                +--rw admin-down?                       boolean
+                +--rw authentication! {authentication}?
+                |  +--rw key-chain?    key-chain:key-chain-ref
+                |  +--rw meticulous?   boolean
+                +--rw use-ipv4?                         boolean
+                +--rw use-ipv6?                         boolean
+                +--ro member-links* [member-link]
+                   +--ro member-link       if:interface-ref
+                   +--ro micro-bfd-ipv4
+                   |  +--ro path-type?              identityref
+                   |  +--ro ip-encapsulation?       boolean
+                   |  +--ro local-discriminator?    discriminator
+                   |  +--ro remote-discriminator?   discriminator
+                   |  +--ro remote-multiplier?      multiplier
+                   |  +--ro demand-capability?      boolean
+                   |  |       {demand-mode}?
+                   |  +--ro source-port?            inet:port-number
+                   |  +--ro dest-port?              inet:port-number
+                   |  +--ro session-running
+                   |  |  +--ro session-index?                uint32
+                   |  |  +--ro local-state?                  state
+                   |  |  +--ro remote-state?                 state
+                   |  |  +--ro local-diagnostic?
+                   |  |  |       iana-bfd-types:diagnostic
+                   |  |  +--ro remote-diagnostic?
+                   |  |  |       iana-bfd-types:diagnostic
+                   |  |  +--ro remote-authenticated?         boolean
+                   |  |  +--ro remote-authentication-type?
+                   |  |  |       iana-bfd-types:auth-type
+                   |  |  |       {authentication}?
+                   |  |  +--ro detection-mode?               enumeration
+                   |  |  +--ro negotiated-tx-interval?       uint32
+                   |  |  +--ro negotiated-rx-interval?       uint32
+                   |  |  +--ro detection-time?               uint32
+                   |  |  +--ro echo-tx-interval-in-use?      uint32
+                   |  |          {echo-mode}?
+                   |  +--ro session-statistics
+                   |     +--ro create-time?
+                   |     |       yang:date-and-time
+                   |     +--ro last-down-time?
+                   |     |       yang:date-and-time
+                   |     +--ro last-up-time?
+                   |     |       yang:date-and-time
+                   |     +--ro down-count?
+                   |     |       yang:counter32
+                   |     +--ro admin-down-count?
+                   |     |       yang:counter32
+                   |     +--ro receive-packet-count?
+                   |     |       yang:counter64
+                   |     +--ro send-packet-count?
+                   |     |       yang:counter64
+                   |     +--ro receive-invalid-packet-count?
+                   |     |       yang:counter64
+                   |     +--ro send-failed-packet-count?
+                   |             yang:counter64
+                   +--ro micro-bfd-ipv6
+                      +--ro path-type?              identityref
+                      +--ro ip-encapsulation?       boolean
+                      +--ro local-discriminator?    discriminator
+                      +--ro remote-discriminator?   discriminator
+                      +--ro remote-multiplier?      multiplier
+                      +--ro demand-capability?      boolean
+                      |       {demand-mode}?
+                      +--ro source-port?            inet:port-number
+                      +--ro dest-port?              inet:port-number
+                      +--ro session-running
+                      |  +--ro session-index?                uint32
+                      |  +--ro local-state?                  state
+                      |  +--ro remote-state?                 state
+                      |  +--ro local-diagnostic?
+                      |  |       iana-bfd-types:diagnostic
+                      |  +--ro remote-diagnostic?
+                      |  |       iana-bfd-types:diagnostic
+                      |  +--ro remote-authenticated?         boolean
+                      |  +--ro remote-authentication-type?
+                      |  |       iana-bfd-types:auth-type
+                      |  |       {authentication}?
+                      |  +--ro detection-mode?               enumeration
+                      |  +--ro negotiated-tx-interval?       uint32
+                      |  +--ro negotiated-rx-interval?       uint32
+                      |  +--ro detection-time?               uint32
+                      |  +--ro echo-tx-interval-in-use?      uint32
+                      |          {echo-mode}?
+                      +--ro session-statistics
+                         +--ro create-time?
+                         |       yang:date-and-time
+                         +--ro last-down-time?
+                         |       yang:date-and-time
+                         +--ro last-up-time?
+                         |       yang:date-and-time
+                         +--ro down-count?
+                         |       yang:counter32
+                         +--ro admin-down-count?
+                         |       yang:counter32
+                         +--ro receive-packet-count?
+                         |       yang:counter64
+                         +--ro send-packet-count?
+                         |       yang:counter64
+                         +--ro receive-invalid-packet-count?
+                         |       yang:counter64
+                         +--ro send-failed-packet-count?
+                                 yang:counter64
+
+     notifications:
+       +---n lag-notification
+          +--ro local-discr?                 discriminator
+          +--ro remote-discr?                discriminator
+          +--ro new-state?                   state
+          +--ro state-change-reason?         iana-bfd-types:diagnostic
+          +--ro time-of-last-state-change?   yang:date-and-time
+          +--ro dest-addr?                   inet:ip-address
+          +--ro source-addr?                 inet:ip-address
+          +--ro session-index?               uint32
+          +--ro path-type?                   identityref
+          +--ro lag-name?                    if:interface-ref
+          +--ro member-link?                 if:interface-ref
+
+2.9.  BFD-over-MPLS-LSPs Hierarchy
+
+   An "mpls" node is added under the "bfd" node in "control-plane-
+   protocol".  The configuration is per MPLS FEC under this "mpls" node.
+   In the operational state model, we support multiple BFD sessions per
+   MPLS FEC (ECMP); the local discriminator is used as the key.  The
+   "mpls" node can be used in a network device (top level) or can be
+   mounted in an LNE or network instance.
+
+   module: ietf-bfd-mpls
+     augment /rt:routing/rt:control-plane-protocols
+               /rt:control-plane-protocol/bfd:bfd:
+       +--rw mpls
+          +--ro summary
+          |  +--ro number-of-sessions?              yang:gauge32
+          |  +--ro number-of-sessions-up?           yang:gauge32
+          |  +--ro number-of-sessions-down?         yang:gauge32
+          |  +--ro number-of-sessions-admin-down?   yang:gauge32
+          +--rw egress
+          |  +--rw enabled?                          boolean
+          |  +--rw local-multiplier?                 multiplier
+          |  +--rw (interval-config-type)?
+          |  |  +--:(tx-rx-intervals)
+          |  |  |  +--rw desired-min-tx-interval?    uint32
+          |  |  |  +--rw required-min-rx-interval?   uint32
+          |  |  +--:(single-interval) {single-minimum-interval}?
+          |  |     +--rw min-interval?               uint32
+          |  +--rw authentication! {authentication}?
+          |     +--rw key-chain?    key-chain:key-chain-ref
+          |     +--rw meticulous?   boolean
+          +--rw session-groups
+             +--rw session-group* [mpls-fec]
+                +--rw mpls-fec                          inet:ip-prefix
+                +--rw local-multiplier?                 multiplier
+                +--rw (interval-config-type)?
+                |  +--:(tx-rx-intervals)
+                |  |  +--rw desired-min-tx-interval?    uint32
+                |  |  +--rw required-min-rx-interval?   uint32
+                |  +--:(single-interval) {single-minimum-interval}?
+                |     +--rw min-interval?               uint32
+                +--rw demand-enabled?                   boolean
+                |       {demand-mode}?
+                +--rw admin-down?                       boolean
+                +--rw authentication! {authentication}?
+                |  +--rw key-chain?    key-chain:key-chain-ref
+                |  +--rw meticulous?   boolean
+                +--ro sessions* []
+                   +--ro path-type?              identityref
+                   +--ro ip-encapsulation?       boolean
+                   +--ro local-discriminator?    discriminator
+                   +--ro remote-discriminator?   discriminator
+                   +--ro remote-multiplier?      multiplier
+                   +--ro demand-capability?      boolean {demand-mode}?
+                   +--ro source-port?            inet:port-number
+                   +--ro dest-port?              inet:port-number
+                   +--ro session-running
+                   |  +--ro session-index?                uint32
+                   |  +--ro local-state?                  state
+                   |  +--ro remote-state?                 state
+                   |  +--ro local-diagnostic?
+                   |  |       iana-bfd-types:diagnostic
+                   |  +--ro remote-diagnostic?
+                   |  |       iana-bfd-types:diagnostic
+                   |  +--ro remote-authenticated?         boolean
+                   |  +--ro remote-authentication-type?
+                   |  |       iana-bfd-types:auth-type {authentication}?
+                   |  +--ro detection-mode?               enumeration
+                   |  +--ro negotiated-tx-interval?       uint32
+                   |  +--ro negotiated-rx-interval?       uint32
+                   |  +--ro detection-time?               uint32
+                   |  +--ro echo-tx-interval-in-use?      uint32
+                   |          {echo-mode}?
+                   +--ro session-statistics
+                   |  +--ro create-time?
+                   |  |       yang:date-and-time
+                   |  +--ro last-down-time?
+                   |  |       yang:date-and-time
+                   |  +--ro last-up-time?
+                   |  |       yang:date-and-time
+                   |  +--ro down-count?
+                   |  |       yang:counter32
+                   |  +--ro admin-down-count?
+                   |  |       yang:counter32
+                   |  +--ro receive-packet-count?
+                   |  |       yang:counter64
+                   |  +--ro send-packet-count?
+                   |  |       yang:counter64
+                   |  +--ro receive-invalid-packet-count?
+                   |  |       yang:counter64
+                   |  +--ro send-failed-packet-count?
+                   |          yang:counter64
+                   +--ro mpls-dest-address?      inet:ip-address
+
+     notifications:
+       +---n mpls-notification
+          +--ro local-discr?                 discriminator
+          +--ro remote-discr?                discriminator
+          +--ro new-state?                   state
+          +--ro state-change-reason?         iana-bfd-types:diagnostic
+          +--ro time-of-last-state-change?   yang:date-and-time
+          +--ro dest-addr?                   inet:ip-address
+          +--ro source-addr?                 inet:ip-address
+          +--ro session-index?               uint32
+          +--ro path-type?                   identityref
+          +--ro mpls-dest-address?           inet:ip-address
+
+2.10.  Interaction with Other YANG Modules
+
+   "Generic YANG Data Model for the Management of Operations,
+   Administration, and Maintenance (OAM) Protocols That Use
+   Connectionless Communications" [RFC8532] describes how the Layer-
+   Independent OAM Management in the Multi-Layer Environment (LIME)
+   connectionless OAM model could be extended to support BFD.
+
+   Also, the operation of the BFD data model depends on configuration
+   parameters that are defined in other YANG modules.
+
+2.10.1.  "ietf-interfaces" Module
+
+   The following boolean configuration is defined in "A YANG Data Model
+   for Interface Management" [RFC8343]:
+
+   /if:interfaces/if:interface/if:enabled
+      If this configuration is set to "false", no BFD packets can be
+      transmitted or received on that interface.
+
+2.10.2.  "ietf-ip" Module
+
+   The following boolean configuration is defined in "A YANG Data Model
+   for IP Management" [RFC8344]:
+
+   /if:interfaces/if:interface/ip:ipv4/ip:enabled
+      If this configuration is set to "false", no BFD IPv4 packets can
+      be transmitted or received on that interface.
+
+   /if:interfaces/if:interface/ip:ipv4/ip:forwarding
+      If this configuration is set to "false", no BFD IPv4 packets can
+      be transmitted or received on that interface.
+
+   /if:interfaces/if:interface/ip:ipv6/ip:enabled
+      If this configuration is set to "false", no BFD IPv6 packets can
+      be transmitted or received on that interface.
+
+   /if:interfaces/if:interface/ip:ipv6/ip:forwarding
+      If this configuration is set to "false", no BFD IPv6 packets can
+      be transmitted or received on that interface.
+
+2.10.3.  "ietf-mpls" Module
+
+   The following boolean configuration is defined in "A YANG Data Model
+   for MPLS Base" [RFC8960]:
+
+   /rt:routing/mpls:mpls/mpls:interfaces/mpls:interface/
+   mpls:mpls-enabled
+      If this configuration is set to "false", no BFD MPLS packets can
+      be transmitted or received on that interface.
+
+2.11.  IANA BFD YANG Module
+
+   This YANG module imports definitions from [RFC5880].  It references
+   [RFC5880] and [RFC6428].
+
+   <CODE BEGINS> file "iana-bfd-types@2021-10-21.yang"
+   module iana-bfd-types {
+     yang-version 1.1;
+     namespace "urn:ietf:params:xml:ns:yang:iana-bfd-types";
+     prefix iana-bfd-types;
+
+     organization
+       "IANA";
+     contact
+       "Internet Assigned Numbers Authority
+
+        Postal: ICANN
+                12025 Waterfront Drive, Suite 300
+                Los Angeles, CA 90094-2536
+                United States of America
+        Tel:    +1 310 301 5800
+        <mailto:iana@iana.org>";
+     description
+       "This module defines YANG data types for IANA-registered
+        BFD parameters.
+
+        This YANG module is maintained by IANA and reflects the
+        'BFD Diagnostic Codes' and 'BFD Authentication Types'
+        registries.
+
+        Copyright (c) 2021 IETF Trust and the persons identified as
+        authors of the code.  All rights reserved.
+
+        Redistribution and use in source and binary forms, with or
+        without modification, is permitted pursuant to, and subject to
+        the license terms contained in, the Simplified BSD License set
+        forth in Section 4.c of the IETF Trust's Legal Provisions
+        Relating to IETF Documents
+        (https://trustee.ietf.org/license-info).
+
+        This version of this YANG module is part of RFC 9127; see the
+        RFC itself for full legal notices.";
+     reference
+       "RFC 9127: YANG Data Model for Bidirectional Forwarding
+        Detection (BFD)";
+
+     revision 2021-10-21 {
+       description
+         "Initial revision.";
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+
+     /*
+      * Type definitions
+      */
+
+     typedef diagnostic {
+       type enumeration {
+         enum none {
+           value 0;
+           description
+             "No Diagnostic.";
+         }
+         enum control-expiry {
+           value 1;
+           description
+             "Control Detection Time Expired.";
+         }
+         enum echo-failed {
+           value 2;
+           description
+             "Echo Function Failed.";
+         }
+         enum neighbor-down {
+           value 3;
+           description
+             "Neighbor Signaled Session Down.";
+         }
+         enum forwarding-reset {
+           value 4;
+           description
+             "Forwarding Plane Reset.";
+         }
+         enum path-down {
+           value 5;
+           description
+             "Path Down.";
+         }
+         enum concatenated-path-down {
+           value 6;
+           description
+             "Concatenated Path Down.";
+         }
+         enum admin-down {
+           value 7;
+           description
+             "Administratively Down.";
+         }
+         enum reverse-concatenated-path-down {
+           value 8;
+           description
+             "Reverse Concatenated Path Down.";
+         }
+         enum mis-connectivity-defect {
+           value 9;
+           description
+             "Mis-connectivity defect.";
+           reference
+             "RFC 5880: Bidirectional Forwarding Detection (BFD)
+              RFC 6428: Proactive Connectivity Verification, Continuity
+              Check, and Remote Defect Indication for the MPLS Transport
+              Profile";
+         }
+       }
+       description
+         "BFD diagnostic codes as defined in RFC 5880.  Values are
+          maintained in the 'BFD Diagnostic Codes' IANA registry.
+          Range is 0 to 31.";
+       reference
+         "RFC 5880: Bidirectional Forwarding Detection (BFD)";
+     }
+
+     typedef auth-type {
+       type enumeration {
+         enum reserved {
+           value 0;
+           description
+             "Reserved.";
+         }
+         enum simple-password {
+           value 1;
+           description
+             "Simple Password.";
+         }
+         enum keyed-md5 {
+           value 2;
+           description
+             "Keyed MD5.";
+         }
+         enum meticulous-keyed-md5 {
+           value 3;
+           description
+             "Meticulous Keyed MD5.";
+         }
+         enum keyed-sha1 {
+           value 4;
+           description
+             "Keyed SHA1.";
+         }
+         enum meticulous-keyed-sha1 {
+           value 5;
+           description
+             "Meticulous Keyed SHA1.";
+         }
+       }
+       description
+         "BFD authentication type as defined in RFC 5880.  Values are
+          maintained in the 'BFD Authentication Types' IANA registry.
+          Range is 0 to 255.";
+       reference
+         "RFC 5880: Bidirectional Forwarding Detection (BFD)";
+     }
+   }
+   <CODE ENDS>
+
+2.12.  BFD Types YANG Module
+
+   This YANG module imports typedefs from [RFC6991] and [RFC8177].  It
+   also imports definitions from [RFC5880], [RFC5881], [RFC5883],
+   [RFC5884], and [RFC7130], as well as the "control-plane-protocol"
+   identity from [RFC8349].
+
+   <CODE BEGINS> file "ietf-bfd-types@2021-10-21.yang"
+   module ietf-bfd-types {
+     yang-version 1.1;
+     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-types";
+     prefix bfd-types;
+
+     import iana-bfd-types {
+       prefix iana-bfd-types;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-inet-types {
+       prefix inet;
+       reference
+         "RFC 6991: Common YANG Data Types";
+     }
+     import ietf-yang-types {
+       prefix yang;
+       reference
+         "RFC 6991: Common YANG Data Types";
+     }
+     import ietf-routing {
+       prefix rt;
+       reference
+         "RFC 8349: A YANG Data Model for Routing Management
+          (NMDA Version)";
+     }
+     import ietf-key-chain {
+       prefix key-chain;
+       reference
+         "RFC 8177: YANG Data Model for Key Chains";
+     }
+
+     organization
+       "IETF BFD Working Group";
+     contact
+       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
+        WG List:  <mailto:rtg-bfd@ietf.org>
+
+        Editor:   Reshad Rahman
+                  <mailto:reshad@yahoo.com>
+
+        Editor:   Lianshu Zheng
+                  <mailto:veronique_cheng@hotmail.com>
+
+        Editor:   Mahesh Jethanandani
+                  <mailto:mjethanandani@gmail.com>";
+     description
+       "This module contains a collection of BFD-specific YANG data type
+        definitions, as per RFC 5880, and also groupings that are common
+        to other BFD YANG modules.
+
+        Copyright (c) 2021 IETF Trust and the persons identified as
+        authors of the code.  All rights reserved.
+
+        Redistribution and use in source and binary forms, with or
+        without modification, is permitted pursuant to, and subject to
+        the license terms contained in, the Simplified BSD License set
+        forth in Section 4.c of the IETF Trust's Legal Provisions
+        Relating to IETF Documents
+        (https://trustee.ietf.org/license-info).
+
+        This version of this YANG module is part of RFC 9127; see the
+        RFC itself for full legal notices.";
+     reference
+       "RFC 5880: Bidirectional Forwarding Detection (BFD)
+        RFC 9127: YANG Data Model for Bidirectional Forwarding
+        Detection (BFD)";
+
+     revision 2021-10-21 {
+       description
+         "Initial revision.";
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+
+     /*
+      * Feature definitions
+      */
+
+     feature single-minimum-interval {
+       description
+         "This feature indicates that the server supports configuration
+          of one minimum interval value that is used for both transmit
+          and receive minimum intervals.";
+     }
+
+     feature authentication {
+       description
+         "This feature indicates that the server supports BFD
+          authentication.";
+       reference
+         "RFC 5880: Bidirectional Forwarding Detection (BFD),
+          Section 6.7";
+     }
+
+     feature demand-mode {
+       description
+         "This feature indicates that the server supports BFD Demand
+          mode.";
+       reference
+         "RFC 5880: Bidirectional Forwarding Detection (BFD),
+          Section 6.6";
+     }
+
+     feature echo-mode {
+       description
+         "This feature indicates that the server supports BFD Echo
+          mode.";
+       reference
+         "RFC 5880: Bidirectional Forwarding Detection (BFD),
+          Section 6.4";
+     }
+
+     /*
+      * Identity definitions
+      */
+
+     identity bfdv1 {
+       base rt:control-plane-protocol;
+       description
+         "BFD protocol version 1.";
+       reference
+         "RFC 5880: Bidirectional Forwarding Detection (BFD)";
+     }
+
+     identity path-type {
+       description
+         "Base identity for the BFD path type.  The path type indicates
+          the type of path on which BFD is running.";
+     }
+
+     identity path-ip-sh {
+       base path-type;
+       description
+         "BFD on IP single-hop.";
+       reference
+         "RFC 5881: Bidirectional Forwarding Detection (BFD)
+          for IPv4 and IPv6 (Single Hop)";
+     }
+
+     identity path-ip-mh {
+       base path-type;
+       description
+         "BFD on IP multihop paths.";
+       reference
+         "RFC 5883: Bidirectional Forwarding Detection (BFD) for
+          Multihop Paths";
+     }
+
+     identity path-mpls-te {
+       base path-type;
+       description
+         "BFD on MPLS Traffic Engineering.";
+       reference
+         "RFC 5884: Bidirectional Forwarding Detection (BFD)
+          for MPLS Label Switched Paths (LSPs)";
+     }
+
+     identity path-mpls-lsp {
+       base path-type;
+       description
+         "BFD on an MPLS Label Switched Path.";
+       reference
+         "RFC 5884: Bidirectional Forwarding Detection (BFD)
+          for MPLS Label Switched Paths (LSPs)";
+     }
+
+     identity path-lag {
+       base path-type;
+       description
+         "Micro-BFD on LAG member links.";
+       reference
+         "RFC 7130: Bidirectional Forwarding Detection (BFD) on
+          Link Aggregation Group (LAG) Interfaces";
+     }
+
+     identity encap-type {
+       description
+         "Base identity for BFD encapsulation type.";
+     }
+
+     identity encap-ip {
+       base encap-type;
+       description
+         "BFD with IP encapsulation.";
+     }
+
+     /*
+      * Type definitions
+      */
+
+     typedef discriminator {
+       type uint32;
+       description
+         "BFD Discriminator as described in RFC 5880.";
+       reference
+         "RFC 5880: Bidirectional Forwarding Detection (BFD)";
+     }
+
+     typedef state {
+       type enumeration {
+         enum adminDown {
+           value 0;
+           description
+             "'adminDown' state.";
+         }
+         enum down {
+           value 1;
+           description
+             "'Down' state.";
+         }
+         enum init {
+           value 2;
+           description
+             "'Init' state.";
+         }
+         enum up {
+           value 3;
+           description
+             "'Up' state.";
+         }
+       }
+       description
+         "BFD states as defined in RFC 5880.";
+     }
+
+     typedef multiplier {
+       type uint8 {
+         range "1..255";
+       }
+       description
+         "BFD multiplier as described in RFC 5880.";
+     }
+
+     typedef hops {
+       type uint8 {
+         range "1..255";
+       }
+       description
+         "This corresponds to Time To Live for IPv4 and corresponds to
+          the hop limit for IPv6.";
+     }
+
+     /*
+      * Groupings
+      */
+
+     grouping auth-parms {
+       description
+         "Grouping for BFD authentication parameters
+          (see Section 6.7 of RFC 5880).";
+       container authentication {
+         if-feature "authentication";
+         presence "Enables BFD authentication (see Section 6.7
+                   of RFC 5880).";
+         description
+           "Parameters for BFD authentication.";
+         reference
+           "RFC 5880: Bidirectional Forwarding Detection (BFD),
+            Section 6.7";
+         leaf key-chain {
+           type key-chain:key-chain-ref;
+           description
+             "Name of the 'key-chain' as per RFC 8177.";
+         }
+         leaf meticulous {
+           type boolean;
+           description
+             "Enables a meticulous mode as per Section 6.7 of
+              RFC 5880.";
+         }
+       }
+     }
+
+     grouping base-cfg-parms {
+       description
+         "BFD grouping for base configuration parameters.";
+       leaf local-multiplier {
+         type multiplier;
+         default "3";
+         description
+           "Multiplier transmitted by the local system.";
+       }
+       choice interval-config-type {
+         default "tx-rx-intervals";
+         description
+           "Two interval values or one value used for both transmit and
+            receive.";
+         case tx-rx-intervals {
+           leaf desired-min-tx-interval {
+             type uint32;
+             units "microseconds";
+             default "1000000";
+             description
+               "Desired minimum transmit interval of control packets.";
+           }
+           leaf required-min-rx-interval {
+             type uint32;
+             units "microseconds";
+             default "1000000";
+             description
+               "Required minimum receive interval of control packets.";
+           }
+         }
+         case single-interval {
+           if-feature "single-minimum-interval";
+           leaf min-interval {
+             type uint32;
+             units "microseconds";
+             default "1000000";
+             description
+               "Desired minimum transmit interval and required
+                minimum receive interval of control packets.";
+           }
+         }
+       }
+     }
+
+     grouping client-cfg-parms {
+       description
+         "BFD grouping for configuration parameters
+          used by BFD clients, e.g., IGP or MPLS.";
+       leaf enabled {
+         type boolean;
+         default "false";
+         description
+           "Indicates whether BFD is enabled.";
+       }
+       uses base-cfg-parms;
+     }
+
+     grouping common-cfg-parms {
+       description
+         "BFD grouping for common configuration parameters.";
+       uses base-cfg-parms;
+       leaf demand-enabled {
+         if-feature "demand-mode";
+         type boolean;
+         default "false";
+         description
+           "To enable Demand mode.";
+       }
+       leaf admin-down {
+         type boolean;
+         default "false";
+         description
+           "Indicates whether the BFD session is administratively
+            down.";
+       }
+       uses auth-parms;
+     }
+
+     grouping all-session {
+       description
+         "BFD session operational information.";
+       leaf path-type {
+         type identityref {
+           base path-type;
+         }
+         config false;
+         description
+           "BFD path type.  This indicates the path type that BFD is
+            running on.";
+       }
+       leaf ip-encapsulation {
+         type boolean;
+         config false;
+         description
+           "Indicates whether BFD encapsulation uses IP.";
+       }
+       leaf local-discriminator {
+         type discriminator;
+         config false;
+         description
+           "Local discriminator.";
+       }
+       leaf remote-discriminator {
+         type discriminator;
+         config false;
+         description
+           "Remote discriminator.";
+       }
+       leaf remote-multiplier {
+         type multiplier;
+         config false;
+         description
+           "Remote multiplier.";
+       }
+       leaf demand-capability {
+         if-feature "demand-mode";
+         type boolean;
+         config false;
+         description
+           "Local Demand mode capability.";
+       }
+       leaf source-port {
+         when "../ip-encapsulation = 'true'" {
+           description
+             "Source port valid only when IP encapsulation is used.";
+         }
+         type inet:port-number;
+         config false;
+         description
+           "Source UDP port.";
+       }
+       leaf dest-port {
+         when "../ip-encapsulation = 'true'" {
+           description
+             "Destination port valid only when IP encapsulation
+              is used.";
+         }
+         type inet:port-number;
+         config false;
+         description
+           "Destination UDP port.";
+       }
+       container session-running {
+         config false;
+         description
+           "BFD 'session-running' information.";
+         leaf session-index {
+           type uint32;
+           description
+             "An index used to uniquely identify BFD sessions.";
+         }
+         leaf local-state {
+           type state;
+           description
+             "Local state.";
+         }
+         leaf remote-state {
+           type state;
+           description
+             "Remote state.";
+         }
+         leaf local-diagnostic {
+           type iana-bfd-types:diagnostic;
+           description
+             "Local diagnostic.";
+         }
+         leaf remote-diagnostic {
+           type iana-bfd-types:diagnostic;
+           description
+             "Remote diagnostic.";
+         }
+         leaf remote-authenticated {
+           type boolean;
+           description
+             "Indicates whether incoming BFD control packets are
+              authenticated.";
+         }
+         leaf remote-authentication-type {
+           when "../remote-authenticated = 'true'" {
+             description
+               "Only valid when incoming BFD control packets are
+                authenticated.";
+           }
+           if-feature "authentication";
+           type iana-bfd-types:auth-type;
+           description
+             "Authentication type of incoming BFD control packets.";
+         }
+         leaf detection-mode {
+           type enumeration {
+             enum async-with-echo {
+               value 1;
+               description
+                 "Async with echo.";
+             }
+             enum async-without-echo {
+               value 2;
+               description
+                 "Async without echo.";
+             }
+             enum demand-with-echo {
+               value 3;
+               description
+                 "Demand with echo.";
+             }
+             enum demand-without-echo {
+               value 4;
+               description
+                 "Demand without echo.";
+             }
+           }
+           description
+             "Detection mode.";
+         }
+         leaf negotiated-tx-interval {
+           type uint32;
+           units "microseconds";
+           description
+             "Negotiated transmit interval.";
+         }
+         leaf negotiated-rx-interval {
+           type uint32;
+           units "microseconds";
+           description
+             "Negotiated receive interval.";
+         }
+         leaf detection-time {
+           type uint32;
+           units "microseconds";
+           description
+             "Detection time.";
+         }
+         leaf echo-tx-interval-in-use {
+           when "../../path-type = 'bfd-types:path-ip-sh'" {
+             description
+               "Echo is supported for IP single-hop only.";
+           }
+           if-feature "echo-mode";
+           type uint32;
+           units "microseconds";
+           description
+             "Echo transmit interval in use.";
+         }
+       }
+       container session-statistics {
+         config false;
+         description
+           "BFD per-session statistics.";
+         leaf create-time {
+           type yang:date-and-time;
+           description
+             "Time and date when this session was created.";
+         }
+         leaf last-down-time {
+           type yang:date-and-time;
+           description
+             "Time and date of the last time this session went down.";
+         }
+         leaf last-up-time {
+           type yang:date-and-time;
+           description
+             "Time and date of the last time this session went up.";
+         }
+         leaf down-count {
+           type yang:counter32;
+           description
+             "The number of times this session has transitioned to the
+              'down' state.";
+         }
+         leaf admin-down-count {
+           type yang:counter32;
+           description
+             "The number of times this session has transitioned to the
+              'admin-down' state.";
+         }
+         leaf receive-packet-count {
+           type yang:counter64;
+           description
+             "Count of received packets in this session.  This includes
+              valid and invalid received packets.";
+         }
+         leaf send-packet-count {
+           type yang:counter64;
+           description
+             "Count of sent packets in this session.";
+         }
+         leaf receive-invalid-packet-count {
+           type yang:counter64;
+           description
+             "Count of invalid received packets in this session.";
+         }
+         leaf send-failed-packet-count {
+           type yang:counter64;
+           description
+             "Count of packets that failed to be sent in this session.";
+         }
+       }
+     }
+
+     grouping session-statistics-summary {
+       description
+         "Grouping for session statistics summary.";
+       container summary {
+         config false;
+         description
+           "BFD session statistics summary.";
+         leaf number-of-sessions {
+           type yang:gauge32;
+           description
+             "Number of BFD sessions.";
+         }
+         leaf number-of-sessions-up {
+           type yang:gauge32;
+           description
+             "Number of BFD sessions currently in the 'Up' state
+              (as defined in RFC 5880).";
+         }
+         leaf number-of-sessions-down {
+           type yang:gauge32;
+           description
+             "Number of BFD sessions currently in the 'Down' or 'Init'
+              state but not 'adminDown' (as defined in RFC 5880).";
+         }
+         leaf number-of-sessions-admin-down {
+           type yang:gauge32;
+           description
+             "Number of BFD sessions currently in the 'adminDown' state
+              (as defined in RFC 5880).";
+         }
+       }
+     }
+
+     grouping notification-parms {
+       description
+         "This group describes common parameters that will be sent
+          as part of BFD notifications.";
+       leaf local-discr {
+         type discriminator;
+         description
+           "BFD local discriminator.";
+       }
+       leaf remote-discr {
+         type discriminator;
+         description
+           "BFD remote discriminator.";
+       }
+       leaf new-state {
+         type state;
+         description
+           "Current BFD state.";
+       }
+       leaf state-change-reason {
+         type iana-bfd-types:diagnostic;
+         description
+           "Reason for the BFD state change.";
+       }
+       leaf time-of-last-state-change {
+         type yang:date-and-time;
+         description
+           "Calendar time of the most recent previous state change.";
+       }
+       leaf dest-addr {
+         type inet:ip-address;
+         description
+           "BFD peer address.";
+       }
+       leaf source-addr {
+         type inet:ip-address;
+         description
+           "BFD local address.";
+       }
+       leaf session-index {
+         type uint32;
+         description
+           "An index used to uniquely identify BFD sessions.";
+       }
+       leaf path-type {
+         type identityref {
+           base path-type;
+         }
+         description
+           "BFD path type.";
+       }
+     }
+   }
+   <CODE ENDS>
+
+2.13.  BFD Top-Level YANG Module
+
+   This YANG module imports and augments "/routing/control-plane-
+   protocols/control-plane-protocol" from [RFC8349].  It also references
+   [RFC5880].
+
+   <CODE BEGINS> file "ietf-bfd@2021-10-21.yang"
+   module ietf-bfd {
+     yang-version 1.1;
+     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd";
+     prefix bfd;
+
+     import ietf-bfd-types {
+       prefix bfd-types;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-routing {
+       prefix rt;
+       reference
+         "RFC 8349: A YANG Data Model for Routing Management
+          (NMDA Version)";
+     }
+
+     organization
+       "IETF BFD Working Group";
+     contact
+       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
+        WG List:  <mailto:rtg-bfd@ietf.org>
+
+        Editor:   Reshad Rahman
+                  <mailto:reshad@yahoo.com>
+
+        Editor:   Lianshu Zheng
+                  <mailto:veronique_cheng@hotmail.com>
+
+        Editor:   Mahesh Jethanandani
+                  <mailto:mjethanandani@gmail.com>";
+     description
+       "This module contains the YANG definition for BFD parameters as
+        per RFC 5880.
+
+        Copyright (c) 2021 IETF Trust and the persons identified as
+        authors of the code.  All rights reserved.
+
+        Redistribution and use in source and binary forms, with or
+        without modification, is permitted pursuant to, and subject to
+        the license terms contained in, the Simplified BSD License set
+        forth in Section 4.c of the IETF Trust's Legal Provisions
+        Relating to IETF Documents
+        (https://trustee.ietf.org/license-info).
+
+        This version of this YANG module is part of RFC 9127; see the
+        RFC itself for full legal notices.";
+     reference
+       "RFC 5880: Bidirectional Forwarding Detection (BFD)
+        RFC 9127: YANG Data Model for Bidirectional Forwarding
+        Detection (BFD)";
+
+     revision 2021-10-21 {
+       description
+         "Initial revision.";
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+
+     augment "/rt:routing/rt:control-plane-protocols/"
+           + "rt:control-plane-protocol" {
+       when "derived-from-or-self(rt:type, 'bfd-types:bfdv1')" {
+         description
+           "This augmentation is only valid for a control-plane protocol
+            instance of BFD (type 'bfdv1').";
+       }
+       description
+         "BFD augmentation.";
+       container bfd {
+         description
+           "BFD top-level container.";
+         uses bfd-types:session-statistics-summary;
+       }
+     }
+   }
+   <CODE ENDS>
+
+2.14.  BFD IP Single-Hop YANG Module
+
+   This YANG module imports "interface-ref" from [RFC8343] and typedefs
+   from [RFC6991].  It also imports and augments "/routing/control-
+   plane-protocols/control-plane-protocol" from [RFC8349], and it
+   references [RFC5881].
+
+   <CODE BEGINS> file "ietf-bfd-ip-sh@2021-10-21.yang"
+   module ietf-bfd-ip-sh {
+     yang-version 1.1;
+     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-ip-sh";
+     prefix bfd-ip-sh;
+
+     import ietf-bfd-types {
+       prefix bfd-types;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-bfd {
+       prefix bfd;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-interfaces {
+       prefix if;
+       reference
+         "RFC 8343: A YANG Data Model for Interface Management";
+     }
+     import ietf-inet-types {
+       prefix inet;
+       reference
+         "RFC 6991: Common YANG Data Types";
+     }
+     import ietf-routing {
+       prefix rt;
+       reference
+         "RFC 8349: A YANG Data Model for Routing Management
+          (NMDA Version)";
+     }
+
+     organization
+       "IETF BFD Working Group";
+     contact
+       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
+        WG List:  <mailto:rtg-bfd@ietf.org>
+
+        Editor:   Reshad Rahman
+                  <mailto:reshad@yahoo.com>
+
+        Editor:   Lianshu Zheng
+                  <mailto:veronique_cheng@hotmail.com>
+
+        Editor:   Mahesh Jethanandani
+                  <mailto:mjethanandani@gmail.com>";
+     description
+       "This module contains the YANG definition for BFD IP single-hop
+        as per RFC 5881.
+
+        Copyright (c) 2021 IETF Trust and the persons identified as
+        authors of the code.  All rights reserved.
+
+        Redistribution and use in source and binary forms, with or
+        without modification, is permitted pursuant to, and subject to
+        the license terms contained in, the Simplified BSD License set
+        forth in Section 4.c of the IETF Trust's Legal Provisions
+        Relating to IETF Documents
+        (https://trustee.ietf.org/license-info).
+
+        This version of this YANG module is part of RFC 9127; see the
+        RFC itself for full legal notices.";
+     reference
+       "RFC 5881: Bidirectional Forwarding Detection (BFD)
+        for IPv4 and IPv6 (Single Hop)
+        RFC 9127: YANG Data Model for Bidirectional Forwarding
+        Detection (BFD)";
+
+     revision 2021-10-21 {
+       description
+         "Initial revision.";
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+
+     /*
+      * Augments
+      */
+
+     augment "/rt:routing/rt:control-plane-protocols/"
+           + "rt:control-plane-protocol/bfd:bfd" {
+       description
+         "BFD augmentation for IP single-hop.";
+       container ip-sh {
+         description
+           "BFD IP single-hop top-level container.";
+         uses bfd-types:session-statistics-summary;
+         container sessions {
+           description
+             "BFD IP single-hop sessions.";
+           list session {
+             key "interface dest-addr";
+             description
+               "List of IP single-hop sessions.";
+             leaf interface {
+               type if:interface-ref;
+               description
+                 "Interface on which the BFD session is running.";
+             }
+             leaf dest-addr {
+               type inet:ip-address;
+               description
+                 "IP address of the peer.";
+             }
+             leaf source-addr {
+               type inet:ip-address;
+               description
+                 "Local IP address.";
+             }
+             uses bfd-types:common-cfg-parms;
+             uses bfd-types:all-session;
+           }
+         }
+         list interfaces {
+           key "interface";
+           description
+             "List of interfaces.";
+           leaf interface {
+             type if:interface-ref;
+             description
+               "BFD information for this interface.";
+           }
+           uses bfd-types:auth-parms;
+         }
+       }
+     }
+
+     /*
+      * Notifications
+      */
+
+     notification singlehop-notification {
+       description
+         "Notification for BFD single-hop session state change.  An
+          implementation may rate-limit notifications, e.g., when a
+          session is continuously changing state.";
+       uses bfd-types:notification-parms;
+       leaf interface {
+         type if:interface-ref;
+         description
+           "Interface to which this BFD session belongs.";
+       }
+       leaf echo-enabled {
+         type boolean;
+         description
+           "Indicates whether Echo was enabled for BFD.";
+       }
+     }
+   }
+   <CODE ENDS>
+
+2.15.  BFD IP Multihop YANG Module
+
+   This YANG module imports typedefs from [RFC6991].  It also imports
+   and augments "/routing/control-plane-protocols/control-plane-
+   protocol" from [RFC8349], and it references [RFC5883].
+
+   <CODE BEGINS> file "ietf-bfd-ip-mh@2021-10-21.yang"
+   module ietf-bfd-ip-mh {
+     yang-version 1.1;
+     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-ip-mh";
+     prefix bfd-ip-mh;
+
+     import ietf-bfd-types {
+       prefix bfd-types;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-bfd {
+       prefix bfd;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-inet-types {
+       prefix inet;
+       reference
+         "RFC 6991: Common YANG Data Types";
+     }
+     import ietf-routing {
+       prefix rt;
+       reference
+         "RFC 8349: A YANG Data Model for Routing Management
+          (NMDA Version)";
+     }
+
+     organization
+       "IETF BFD Working Group";
+     contact
+       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
+        WG List:  <mailto:rtg-bfd@ietf.org>
+
+        Editor:   Reshad Rahman
+                  <mailto:reshad@yahoo.com>
+
+        Editor:   Lianshu Zheng
+                  <mailto:veronique_cheng@hotmail.com>
+
+        Editor:   Mahesh Jethanandani
+                  <mailto:mjethanandani@gmail.com>";
+     description
+       "This module contains the YANG definition for BFD IP multihop
+        as per RFC 5883.
+
+        Copyright (c) 2021 IETF Trust and the persons identified as
+        authors of the code.  All rights reserved.
+
+        Redistribution and use in source and binary forms, with or
+        without modification, is permitted pursuant to, and subject to
+        the license terms contained in, the Simplified BSD License set
+        forth in Section 4.c of the IETF Trust's Legal Provisions
+        Relating to IETF Documents
+        (https://trustee.ietf.org/license-info).
+
+        This version of this YANG module is part of RFC 9127; see the
+        RFC itself for full legal notices.";
+     reference
+       "RFC 5883: Bidirectional Forwarding Detection (BFD) for
+        Multihop Paths
+        RFC 9127: YANG Data Model for Bidirectional Forwarding
+        Detection (BFD)";
+
+     revision 2021-10-21 {
+       description
+         "Initial revision.";
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+
+     /*
+      * Augments
+      */
+
+     augment "/rt:routing/rt:control-plane-protocols/"
+           + "rt:control-plane-protocol/bfd:bfd" {
+       description
+         "BFD augmentation for IP multihop.";
+       container ip-mh {
+         description
+           "BFD IP multihop top-level container.";
+         uses bfd-types:session-statistics-summary;
+         container session-groups {
+           description
+             "BFD IP multihop session groups.";
+           list session-group {
+             key "source-addr dest-addr";
+             description
+               "Group of BFD IP multihop sessions (for ECMP).  A
+                group of sessions is between one source and one
+                destination.  Each session has a different field
+                in the UDP/IP header for ECMP.";
+             leaf source-addr {
+               type inet:ip-address;
+               description
+                 "Local IP address.";
+             }
+             leaf dest-addr {
+               type inet:ip-address;
+               description
+                 "IP address of the peer.";
+             }
+             uses bfd-types:common-cfg-parms;
+             leaf tx-ttl {
+               type bfd-types:hops;
+               default "255";
+               description
+                 "Hop count of outgoing BFD control packets.";
+             }
+             leaf rx-ttl {
+               type bfd-types:hops;
+               mandatory true;
+               description
+                 "Minimum allowed hop count value for incoming BFD
+                  control packets.  Control packets whose hop count is
+                  lower than this value are dropped.";
+             }
+             list sessions {
+               config false;
+               description
+                 "The multiple BFD sessions between a source and a
+                  destination.";
+               uses bfd-types:all-session;
+             }
+           }
+         }
+       }
+     }
+
+     /*
+      * Notifications
+      */
+
+     notification multihop-notification {
+       description
+         "Notification for BFD multihop session state change.  An
+          implementation may rate-limit notifications, e.g., when a
+          session is continuously changing state.";
+       uses bfd-types:notification-parms;
+     }
+   }
+   <CODE ENDS>
+
+2.16.  BFD-over-LAG YANG Module
+
+   This YANG module imports "interface-ref" from [RFC8343] and typedefs
+   from [RFC6991].  It also imports and augments "/routing/control-
+   plane-protocols/control-plane-protocol" from [RFC8349].
+   Additionally, it references [RFC7130].
+
+   <CODE BEGINS> file "ietf-bfd-lag@2021-10-21.yang"
+   module ietf-bfd-lag {
+     yang-version 1.1;
+     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-lag";
+     prefix bfd-lag;
+
+     import ietf-bfd-types {
+       prefix bfd-types;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-bfd {
+       prefix bfd;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-interfaces {
+       prefix if;
+       reference
+         "RFC 8343: A YANG Data Model for Interface Management";
+     }
+     import ietf-inet-types {
+       prefix inet;
+       reference
+         "RFC 6991: Common YANG Data Types";
+     }
+     import ietf-routing {
+       prefix rt;
+       reference
+         "RFC 8349: A YANG Data Model for Routing Management
+          (NMDA Version)";
+     }
+
+     organization
+       "IETF BFD Working Group";
+     contact
+       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
+        WG List:  <mailto:rtg-bfd@ietf.org>
+
+        Editor:   Reshad Rahman
+                  <mailto:reshad@yahoo.com>
+
+        Editor:   Lianshu Zheng
+                  <mailto:veronique_cheng@hotmail.com>
+
+        Editor:   Mahesh Jethanandani
+                  <mailto:mjethanandani@gmail.com>";
+     description
+       "This module contains the YANG definition for BFD-over-LAG
+        interfaces as per RFC 7130.
+
+        Copyright (c) 2021 IETF Trust and the persons identified as
+        authors of the code.  All rights reserved.
+
+        Redistribution and use in source and binary forms, with or
+        without modification, is permitted pursuant to, and subject to
+        the license terms contained in, the Simplified BSD License set
+        forth in Section 4.c of the IETF Trust's Legal Provisions
+        Relating to IETF Documents
+        (https://trustee.ietf.org/license-info).
+
+        This version of this YANG module is part of RFC 9127; see the
+        RFC itself for full legal notices.";
+     reference
+       "RFC 7130: Bidirectional Forwarding Detection (BFD) on
+        Link Aggregation Group (LAG) Interfaces
+        RFC 9127: YANG Data Model for Bidirectional Forwarding
+        Detection (BFD)";
+
+     revision 2021-10-21 {
+       description
+         "Initial revision.";
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+
+     /*
+      * Augments
+      */
+
+     augment "/rt:routing/rt:control-plane-protocols/"
+           + "rt:control-plane-protocol/bfd:bfd" {
+       description
+         "BFD augmentation for a LAG.";
+       container lag {
+         description
+           "BFD-over-LAG top-level container.";
+         container micro-bfd-ipv4-session-statistics {
+           description
+             "Micro-BFD IPv4 session counters.";
+           uses bfd-types:session-statistics-summary;
+         }
+         container micro-bfd-ipv6-session-statistics {
+           description
+             "Micro-BFD IPv6 session counters.";
+           uses bfd-types:session-statistics-summary;
+         }
+         container sessions {
+           description
+             "BFD-over-LAG sessions.";
+           list session {
+             key "lag-name";
+             description
+               "List of BFD-over-LAG sessions.";
+             leaf lag-name {
+               type if:interface-ref;
+               description
+                 "Name of the LAG.";
+             }
+             leaf ipv4-dest-addr {
+               type inet:ipv4-address;
+               description
+                 "IPv4 address of the peer, for IPv4 micro-BFD.";
+             }
+             leaf ipv6-dest-addr {
+               type inet:ipv6-address;
+               description
+                 "IPv6 address of the peer, for IPv6 micro-BFD.";
+             }
+             uses bfd-types:common-cfg-parms;
+             leaf use-ipv4 {
+               type boolean;
+               description
+                 "Using IPv4 micro-BFD.";
+             }
+             leaf use-ipv6 {
+               type boolean;
+               description
+                 "Using IPv6 micro-BFD.";
+             }
+             list member-links {
+               key "member-link";
+               config false;
+               description
+                 "Micro-BFD over a LAG.  This represents one
+                  member link.";
+               leaf member-link {
+                 type if:interface-ref;
+                 description
+                   "Member link on which micro-BFD is running.";
+               }
+               container micro-bfd-ipv4 {
+                 when "../../use-ipv4 = 'true'" {
+                   description
+                     "Needed only if IPv4 is used.";
+                 }
+                 description
+                   "Micro-BFD IPv4 session state on a member link.";
+                 uses bfd-types:all-session;
+               }
+               container micro-bfd-ipv6 {
+                 when "../../use-ipv6 = 'true'" {
+                   description
+                     "Needed only if IPv6 is used.";
+                 }
+                 description
+                   "Micro-BFD IPv6 session state on a member link.";
+                 uses bfd-types:all-session;
+               }
+             }
+           }
+         }
+       }
+     }
+
+     /*
+      * Notifications
+      */
+
+     notification lag-notification {
+       description
+         "Notification for BFD-over-LAG session state change.
+          An implementation may rate-limit notifications, e.g., when a
+          session is continuously changing state.";
+       uses bfd-types:notification-parms;
+       leaf lag-name {
+         type if:interface-ref;
+         description
+           "LAG interface name.";
+       }
+       leaf member-link {
+         type if:interface-ref;
+         description
+           "Member link on which BFD is running.";
+       }
+     }
+   }
+   <CODE ENDS>
+
+2.17.  BFD-over-MPLS YANG Module
+
+   This YANG module imports typedefs from [RFC6991].  It also imports
+   and augments "/routing/control-plane-protocols/control-plane-
+   protocol" from [RFC8349].  Additionally, it references [RFC5586] and
+   [RFC5884].
+
+   <CODE BEGINS> file "ietf-bfd-mpls@2021-10-21.yang"
+   module ietf-bfd-mpls {
+     yang-version 1.1;
+     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-mpls";
+     prefix bfd-mpls;
+
+     import ietf-bfd-types {
+       prefix bfd-types;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-bfd {
+       prefix bfd;
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+     import ietf-inet-types {
+       prefix inet;
+       reference
+         "RFC 6991: Common YANG Data Types";
+     }
+     import ietf-routing {
+       prefix rt;
+       reference
+         "RFC 8349: A YANG Data Model for Routing Management
+          (NMDA Version)";
+     }
+
+     organization
+       "IETF BFD Working Group";
+     contact
+       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
+        WG List:  <mailto:rtg-bfd@ietf.org>
+
+        Editor:   Reshad Rahman
+                  <mailto:reshad@yahoo.com>
+
+        Editor:   Lianshu Zheng
+                  <mailto:veronique_cheng@hotmail.com>
+
+        Editor:   Mahesh Jethanandani
+                  <mailto:mjethanandani@gmail.com>";
+     description
+       "This module contains the YANG definition for BFD parameters for
+        MPLS LSPs as per RFC 5884.
+
+        Copyright (c) 2021 IETF Trust and the persons identified as
+        authors of the code.  All rights reserved.
+
+        Redistribution and use in source and binary forms, with or
+        without modification, is permitted pursuant to, and subject to
+        the license terms contained in, the Simplified BSD License set
+        forth in Section 4.c of the IETF Trust's Legal Provisions
+        Relating to IETF Documents
+        (https://trustee.ietf.org/license-info).
+
+        This version of this YANG module is part of RFC 9127; see the
+        RFC itself for full legal notices.";
+     reference
+       "RFC 5884: Bidirectional Forwarding Detection (BFD)
+        for MPLS Label Switched Paths (LSPs)
+        RFC 9127: YANG Data Model for Bidirectional Forwarding
+        Detection (BFD)";
+
+     revision 2021-10-21 {
+       description
+         "Initial revision.";
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+
+     /*
+      * Identity definitions
+      */
+
+     identity encap-gach {
+       base bfd-types:encap-type;
+       description
+         "BFD with G-ACh encapsulation as per RFC 5586.";
+       reference
+         "RFC 5586: MPLS Generic Associated Channel";
+     }
+
+     identity encap-ip-gach {
+       base bfd-types:encap-type;
+       description
+         "BFD with IP and G-ACh encapsulation as per RFC 5586.";
+     }
+
+     /*
+      * Groupings
+      */
+
+     grouping encap-cfg {
+       description
+         "Configuration for BFD encapsulation.";
+       leaf encap {
+         type identityref {
+           base bfd-types:encap-type;
+         }
+         default "bfd-types:encap-ip";
+         description
+           "BFD encapsulation.";
+       }
+     }
+
+     grouping mpls-dest-address {
+       description
+         "Destination address as per RFC 5884.";
+       reference
+         "RFC 5884: Bidirectional Forwarding Detection (BFD)
+          for MPLS Label Switched Paths (LSPs)";
+       leaf mpls-dest-address {
+         type inet:ip-address;
+         config false;
+         description
+           "Destination address as per RFC 5884.
+            Needed if IP encapsulation is used.";
+       }
+     }
+
+     /*
+      * Augments
+      */
+
+     augment "/rt:routing/rt:control-plane-protocols/"
+           + "rt:control-plane-protocol/bfd:bfd" {
+       description
+         "BFD augmentation for MPLS.";
+       container mpls {
+         description
+           "BFD MPLS top-level container.";
+         uses bfd-types:session-statistics-summary;
+         container egress {
+           description
+             "Egress configuration.";
+           uses bfd-types:client-cfg-parms;
+           uses bfd-types:auth-parms;
+         }
+         container session-groups {
+           description
+             "BFD-over-MPLS session groups.";
+           list session-group {
+             key "mpls-fec";
+             description
+               "Group of BFD MPLS sessions (for ECMP).  A group of
+                sessions is for one FEC.  Each session has a different
+                field in the UDP/IP header for ECMP.";
+             leaf mpls-fec {
+               type inet:ip-prefix;
+               description
+                 "MPLS FEC.";
+             }
+             uses bfd-types:common-cfg-parms;
+             list sessions {
+               config false;
+               description
+                 "The BFD sessions for an MPLS FEC.  The local
+                  discriminator is unique for each session in the
+                  group.";
+               uses bfd-types:all-session;
+               uses bfd-mpls:mpls-dest-address;
+             }
+           }
+         }
+       }
+     }
+
+     /*
+      * Notifications
+      */
+
+     notification mpls-notification {
+       description
+         "Notification for BFD-over-MPLS FEC session state change.
+          An implementation may rate-limit notifications, e.g., when a
+          session is continuously changing state.";
+       uses bfd-types:notification-parms;
+       leaf mpls-dest-address {
+         type inet:ip-address;
+         description
+           "Destination address as per RFC 5884.
+            Needed if IP encapsulation is used.";
+       }
+     }
+   }
+   <CODE ENDS>
+
+3.  Data Model Examples
+
+   This section presents some simple and illustrative examples of how to
+   configure BFD.
+
+   The examples are represented in XML [W3C.REC-xml-20081126].
+
+3.1.  IP Single-Hop
+
+   The following is an example configuration for a BFD IP single-hop
+   session.  The desired transmit interval and the required receive
+   interval are both set to 10 ms.
+
+   <?xml version="1.0" encoding="UTF-8"?>
+   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
+     <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
+       <interface>
+         <name>eth0</name>
+         <type xmlns:ianaift="urn:ietf:params:xml:ns:yang:iana-if-type">
+           ianaift:ethernetCsmacd
+         </type>
+       </interface>
+     </interfaces>
+     <routing xmlns="urn:ietf:params:xml:ns:yang:ietf-routing">
+       <control-plane-protocols>
+         <control-plane-protocol>
+           <type xmlns:bfd-types=
+               "urn:ietf:params:xml:ns:yang:ietf-bfd-types">
+             bfd-types:bfdv1
+           </type>
+           <name>name:BFD</name>
+           <bfd xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd">
+             <ip-sh xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd-ip-sh">
+               <sessions>
+                 <session>
+                   <interface>eth0</interface>
+                   <dest-addr>2001:db8:0:113::101</dest-addr>
+                   <desired-min-tx-interval>
+                     10000
+                   </desired-min-tx-interval>
+                   <required-min-rx-interval>
+                     10000
+                   </required-min-rx-interval>
+                 </session>
+               </sessions>
+             </ip-sh>
+           </bfd>
+         </control-plane-protocol>
+       </control-plane-protocols>
+     </routing>
+   </config>
+
+3.2.  IP Multihop
+
+   The following is an example configuration for a BFD IP multihop
+   session group.  The desired transmit interval and the required
+   receive interval are both set to 150 ms.
+
+   <?xml version="1.0" encoding="UTF-8"?>
+   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
+     <routing xmlns="urn:ietf:params:xml:ns:yang:ietf-routing">
+       <control-plane-protocols>
+         <control-plane-protocol>
+           <type xmlns:bfd-types=
+               "urn:ietf:params:xml:ns:yang:ietf-bfd-types">
+             bfd-types:bfdv1
+           </type>
+           <name>name:BFD</name>
+           <bfd xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd">
+             <ip-mh xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd-ip-mh">
+               <session-groups>
+                 <session-group>
+                   <source-addr>2001:db8:0:113::103</source-addr>
+                   <dest-addr>2001:db8:0:114::100</dest-addr>
+                   <desired-min-tx-interval>
+                     150000
+                   </desired-min-tx-interval>
+                   <required-min-rx-interval>
+                     150000
+                   </required-min-rx-interval>
+                   <rx-ttl>240</rx-ttl>
+                 </session-group>
+               </session-groups>
+             </ip-mh>
+           </bfd>
+         </control-plane-protocol>
+       </control-plane-protocols>
+     </routing>
+   </config>
+
+3.3.  LAG
+
+   The following is an example of BFD configuration for a LAG session.
+   In this case, an interface named "Bundle-Ether1" of interface type
+   "ieee8023adLag" has a desired transmit interval and required receive
+   interval set to 10 ms.
+
+   <?xml version="1.0" encoding="UTF-8"?>
+   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
+     <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
+       <interface>
+         <name>Bundle-Ether1</name>
+         <type xmlns:ianaift="urn:ietf:params:xml:ns:yang:iana-if-type">
+           ianaift:ieee8023adLag
+         </type>
+       </interface>
+     </interfaces>
+     <routing xmlns="urn:ietf:params:xml:ns:yang:ietf-routing">
+       <control-plane-protocols>
+         <control-plane-protocol>
+           <type xmlns:bfd-types=
+               "urn:ietf:params:xml:ns:yang:ietf-bfd-types">
+             bfd-types:bfdv1
+           </type>
+           <name>name:BFD</name>
+           <bfd xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd">
+             <lag xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd-lag">
+               <sessions>
+                 <session>
+                   <lag-name>Bundle-Ether1</lag-name>
+                   <ipv6-dest-addr>2001:db8:112::16</ipv6-dest-addr>
+                   <desired-min-tx-interval>
+                     100000
+                   </desired-min-tx-interval>
+                   <required-min-rx-interval>
+                     100000
+                   </required-min-rx-interval>
+                   <use-ipv6>true</use-ipv6>
+                 </session>
+               </sessions>
+             </lag>
+           </bfd>
+         </control-plane-protocol>
+       </control-plane-protocols>
+     </routing>
+   </config>
+
+3.4.  MPLS
+
+   The following is an example of BFD configured for an MPLS LSP.  In
+   this case, the desired transmit interval and required receive
+   interval are both set to 250 ms.
+
+   <?xml version="1.0" encoding="UTF-8"?>
+   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
+     <routing xmlns="urn:ietf:params:xml:ns:yang:ietf-routing">
+       <control-plane-protocols>
+         <control-plane-protocol>
+           <type xmlns:bfd-types=
+               "urn:ietf:params:xml:ns:yang:ietf-bfd-types">
+             bfd-types:bfdv1
+           </type>
+           <name>name:BFD</name>
+           <bfd xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd">
+             <mpls xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd-mpls">
+               <session-groups>
+                 <session-group>
+                   <mpls-fec>2001:db8:114::/116</mpls-fec>
+                   <desired-min-tx-interval>
+                     250000
+                   </desired-min-tx-interval>
+                   <required-min-rx-interval>
+                     250000
+                   </required-min-rx-interval>
+                 </session-group>
+               </session-groups>
+             </mpls>
+           </bfd>
+         </control-plane-protocol>
+       </control-plane-protocols>
+     </routing>
+   </config>
+
+4.  Security Considerations
+
+   The YANG modules specified in this document define a schema for data
+   that is designed to be accessed via network management protocols such
+   as NETCONF [RFC6241] or RESTCONF [RFC8040].  The lowest NETCONF layer
+   is the secure transport layer, and the mandatory-to-implement secure
+   transport is Secure Shell (SSH) [RFC6242].  The lowest RESTCONF layer
+   is HTTPS, and the mandatory-to-implement secure transport is TLS
+   [RFC8446].
+
+   The Network Configuration Access Control Model (NACM) [RFC8341]
+   provides the means to restrict access for particular NETCONF or
+   RESTCONF users to a preconfigured subset of all available NETCONF or
+   RESTCONF protocol operations and content.
+
+   There are a number of data nodes defined in these YANG modules that
+   are writable/creatable/deletable (i.e., config true, which is the
+   default).  These data nodes may be considered sensitive or vulnerable
+   in some network environments.  Write operations (e.g., edit-config)
+   to these data nodes without proper protection can have a negative
+   effect on network operations.  These are the subtrees and data nodes
+   and their sensitivity/vulnerability from a write access perspective:
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-sh/
+   sessions:
+      This list specifies the IP single-hop BFD sessions.
+
+      Data nodes "local-multiplier", "desired-min-tx-interval",
+      "required-min-rx-interval", and "min-interval" all impact the BFD
+      IP single-hop session.  The "source-addr" and "dest-addr" data
+      nodes can be used to send BFD packets to unwitting recipients.
+      [RFC5880] describes how BFD mitigates such threats.
+      Authentication data nodes "key-chain" and "meticulous" impact the
+      security of the BFD IP single-hop session.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-mh/
+   session-group:
+      This list specifies the IP multihop BFD session groups.
+
+      Data nodes "local-multiplier", "desired-min-tx-interval",
+      "required-min-rx-interval", and "min-interval" all impact the BFD
+      IP multihop session.  The "source-addr" and "dest-addr" data nodes
+      can be used to send BFD packets to unwitting recipients.
+      [RFC5880] describes how BFD mitigates such threats.
+      Authentication data nodes "key-chain" and "meticulous" impact the
+      security of the BFD IP multihop session.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/lag/
+   sessions:
+      This list specifies the BFD sessions over a LAG.
+
+      Data nodes "local-multiplier", "desired-min-tx-interval",
+      "required-min-rx-interval", and "min-interval" all impact the BFD-
+      over-LAG session.  The "ipv4-dest-addr" and "ipv6-dest-addr" data
+      nodes can be used to send BFD packets to unwitting recipients.
+      [RFC5880] describes how BFD mitigates such threats.
+      Authentication data nodes "key-chain" and "meticulous" impact the
+      security of the BFD-over-LAG session.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
+   session-group:
+      This list specifies the session groups for BFD over MPLS.
+
+      Data nodes "local-multiplier", "desired-min-tx-interval",
+      "required-min-rx-interval", and "min-interval" all impact the BFD-
+      over-MPLS-LSPs session.  Authentication data nodes "key-chain" and
+      "meticulous" impact the security of the BFD-over-MPLS-LSPs
+      session.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
+   egress:
+      Data nodes "local-multiplier", "desired-min-tx-interval",
+      "required-min-rx-interval", and "min-interval" all impact the BFD-
+      over-MPLS-LSPs sessions for which this device is an MPLS LSP
+      egress node.  Authentication data nodes "key-chain" and
+      "meticulous" impact the security of the BFD-over-MPLS-LSPs
+      sessions for which this device is an MPLS LSP egress node.
+
+   The YANG modules have writable data nodes that can be used for the
+   creation of BFD sessions and the modification of BFD session
+   parameters.  The system should "police" the creation of BFD sessions
+   to prevent new sessions from causing existing BFD sessions to fail.
+   In the case of BFD session modification, the BFD protocol has
+   mechanisms in place that allow for in-service modification.
+
+   When BFD clients are used to modify BFD configuration (as described
+   in Section 2.1), the BFD clients need to be included in an analysis
+   of the security properties of the system that uses BFD (e.g., when
+   considering the authentication and authorization of control actions).
+   In many cases, BFD is not the most vulnerable portion of such a
+   composite system, since BFD is limited to generating well-defined
+   traffic at a fixed rate on a given path; in the case of an IGP acting
+   as a BFD client, attacking the IGP could cause more broad-scale
+   disruption than would (de)configuring a BFD session.
+
+   Some of the readable data nodes in these YANG modules may be
+   considered sensitive or vulnerable in some network environments.  It
+   is thus important to control read access (e.g., via get, get-config,
+   or notification) to these data nodes.  These are the subtrees and
+   data nodes and their sensitivity/vulnerability from a read access
+   perspective:
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-sh/
+   summary:
+      Access to this information discloses the number of BFD IP single-
+      hop sessions that are in the "up", "down", or "admin-down" state.
+      The counters include BFD sessions for which the user does not have
+      read access.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-
+   sh/sessions/session/:
+      Access to data nodes "local-discriminator" and "remote-
+      discriminator" (combined with the data nodes in the authentication
+      container) provides the ability to spoof BFD IP single-hop
+      packets.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-mh/
+   summary:
+      Access to this information discloses the number of BFD IP multihop
+      sessions that are in the "up", "down", or "admin-down" state.  The
+      counters include BFD sessions for which the user does not have
+      read access.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-mh/
+   session-groups/session-group/sessions:
+      Access to data nodes "local-discriminator" and "remote-
+      discriminator" (combined with the data nodes in the session
+      group's authentication container) provides the ability to spoof
+      BFD IP multihop packets.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/lag/
+   micro-bfd-ipv4-session-statistics/summary:
+      Access to this information discloses the number of micro-BFD IPv4
+      LAG sessions that are in the "up", "down", or "admin-down" state.
+      The counters include BFD sessions for which the user does not have
+      read access.
+
+   /routing/control-plane-protocols/control-plane-
+   protocol/bfd/lag/sessions/session/member-links/member-link/micro-
+   bfd-ipv4:
+      Access to data nodes "local-discriminator" and "remote-
+      discriminator" (combined with the data nodes in the session's
+      authentication container) provides the ability to spoof BFD IPv4
+      LAG packets.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/lag/
+   micro-bfd-ipv6-session-statistics/summary:
+      Access to this information discloses the number of micro-BFD IPv6
+      LAG sessions that are in the "up", "down", or "admin-down" state.
+      The counters include BFD sessions for which the user does not have
+      read access.
+
+   /routing/control-plane-protocols/control-plane-
+   protocol/bfd/lag/sessions/session/member-links/member-link/micro-
+   bfd-ipv6:
+      Access to data nodes "local-discriminator" and "remote-
+      discriminator" (combined with the data nodes in the session's
+      authentication container) provides the ability to spoof BFD IPv6
+      LAG packets.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
+   summary:
+      Access to this information discloses the number of BFD sessions
+      over MPLS LSPs that are in the "up", "down", or "admin-down"
+      state.  The counters include BFD sessions for which the user does
+      not have read access.
+
+   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
+   session-groups/session-group/sessions:
+      Access to data nodes "local-discriminator" and "remote-
+      discriminator" (combined with the data nodes in the session
+      group's authentication container) provides the ability to spoof
+      BFD-over-MPLS-LSPs packets.
+
+   This document does not define any RPC operations.
+
+5.  IANA Considerations
+
+   IANA has registered the following namespace URIs in the "IETF XML
+   Registry" [RFC3688]:
+
+   URI:  urn:ietf:params:xml:ns:yang:iana-bfd-types
+   Registrant Contact:  The IESG.
+   XML:  N/A; the requested URI is an XML namespace.
+
+   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-types
+   Registrant Contact:  The IESG.
+   XML:  N/A; the requested URI is an XML namespace.
+
+   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd
+   Registrant Contact:  The IESG.
+   XML:  N/A; the requested URI is an XML namespace.
+
+   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-ip-sh
+   Registrant Contact:  The IESG.
+   XML:  N/A; the requested URI is an XML namespace.
+
+   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-ip-mh
+   Registrant Contact:  The IESG.
+   XML:  N/A; the requested URI is an XML namespace.
+
+   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-lag
+   Registrant Contact:  The IESG.
+   XML:  N/A; the requested URI is an XML namespace.
+
+   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-mpls
+   Registrant Contact:  The IESG.
+   XML:  N/A; the requested URI is an XML namespace.
+
+   IANA has registered the following YANG modules in the "YANG Module
+   Names" registry [RFC6020]:
+
+   Name:  iana-bfd-types
+   Namespace:  urn:ietf:params:xml:ns:yang:iana-bfd-types
+   Prefix:  iana-bfd-types
+   Reference:  RFC 9127
+
+   Name:  ietf-bfd-types
+   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-types
+   Prefix:  bfd-types
+   Reference:  RFC 9127
+
+   Name:  ietf-bfd
+   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd
+   Prefix:  bfd
+   Reference:  RFC 9127
+
+   Name:  ietf-bfd-ip-sh
+   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-ip-sh
+   Prefix:  bfd-ip-sh
+   Reference:  RFC 9127
+
+   Name:  ietf-bfd-ip-mh
+   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-ip-mh
+   Prefix:  bfd-ip-mh
+   Reference:  RFC 9127
+
+   Name:  ietf-bfd-lag
+   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-lag
+   Prefix:  bfd-lag
+   Reference:  RFC 9127
+
+   Name:  ietf-bfd-mpls
+   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-mpls
+   Prefix:  bfd-mpls
+   Reference:  RFC 9127
+
+5.1.  IANA-Maintained "iana-bfd-types" Module
+
+   This document defines the initial version of the IANA-maintained
+   "iana-bfd-types" YANG module.
+
+   The "iana-bfd-types" YANG module mirrors the "BFD Diagnostic Codes"
+   and "BFD Authentication Types" registries at
+   <https://www.iana.org/assignments/bfd-parameters/>.  Whenever these
+   registries change, IANA must update the "iana-bfd-types" YANG module.
+
+6.  References
+
+6.1.  Normative References
+
+   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
+              DOI 10.17487/RFC3688, January 2004,
+              <https://www.rfc-editor.org/info/rfc3688>.
+
+   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
+              "MPLS Generic Associated Channel", RFC 5586,
+              DOI 10.17487/RFC5586, June 2009,
+              <https://www.rfc-editor.org/info/rfc5586>.
+
+   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
+              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
+              <https://www.rfc-editor.org/info/rfc5880>.
+
+   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
+              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
+              DOI 10.17487/RFC5881, June 2010,
+              <https://www.rfc-editor.org/info/rfc5881>.
+
+   [RFC5882]  Katz, D. and D. Ward, "Generic Application of
+              Bidirectional Forwarding Detection (BFD)", RFC 5882,
+              DOI 10.17487/RFC5882, June 2010,
+              <https://www.rfc-editor.org/info/rfc5882>.
+
+   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
+              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
+              June 2010, <https://www.rfc-editor.org/info/rfc5883>.
+
+   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
+              "Bidirectional Forwarding Detection (BFD) for MPLS Label
+              Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
+              June 2010, <https://www.rfc-editor.org/info/rfc5884>.
+
+   [RFC5885]  Nadeau, T., Ed. and C. Pignataro, Ed., "Bidirectional
+              Forwarding Detection (BFD) for the Pseudowire Virtual
+              Circuit Connectivity Verification (VCCV)", RFC 5885,
+              DOI 10.17487/RFC5885, June 2010,
+              <https://www.rfc-editor.org/info/rfc5885>.
+
+   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
+              the Network Configuration Protocol (NETCONF)", RFC 6020,
+              DOI 10.17487/RFC6020, October 2010,
+              <https://www.rfc-editor.org/info/rfc6020>.
+
+   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
+              and A. Bierman, Ed., "Network Configuration Protocol
+              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
+              <https://www.rfc-editor.org/info/rfc6241>.
+
+   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
+              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
+              <https://www.rfc-editor.org/info/rfc6242>.
+
+   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
+              RFC 6991, DOI 10.17487/RFC6991, July 2013,
+              <https://www.rfc-editor.org/info/rfc6991>.
+
+   [RFC7130]  Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
+              Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
+              Forwarding Detection (BFD) on Link Aggregation Group (LAG)
+              Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
+              2014, <https://www.rfc-editor.org/info/rfc7130>.
+
+   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
+              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
+              <https://www.rfc-editor.org/info/rfc8040>.
+
+   [RFC8177]  Lindem, A., Ed., Qu, Y., Yeung, D., Chen, I., and J.
+              Zhang, "YANG Data Model for Key Chains", RFC 8177,
+              DOI 10.17487/RFC8177, June 2017,
+              <https://www.rfc-editor.org/info/rfc8177>.
+
+   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
+              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
+              <https://www.rfc-editor.org/info/rfc8340>.
+
+   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
+              Access Control Model", STD 91, RFC 8341,
+              DOI 10.17487/RFC8341, March 2018,
+              <https://www.rfc-editor.org/info/rfc8341>.
+
+   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
+              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
+              <https://www.rfc-editor.org/info/rfc8343>.
+
+   [RFC8344]  Bjorklund, M., "A YANG Data Model for IP Management",
+              RFC 8344, DOI 10.17487/RFC8344, March 2018,
+              <https://www.rfc-editor.org/info/rfc8344>.
+
+   [RFC8349]  Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for
+              Routing Management (NMDA Version)", RFC 8349,
+              DOI 10.17487/RFC8349, March 2018,
+              <https://www.rfc-editor.org/info/rfc8349>.
+
+   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
+              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
+              <https://www.rfc-editor.org/info/rfc8446>.
+
+   [RFC8960]  Saad, T., Raza, K., Gandhi, R., Liu, X., and V. Beeram, "A
+              YANG Data Model for MPLS Base", RFC 8960,
+              DOI 10.17487/RFC8960, December 2020,
+              <https://www.rfc-editor.org/info/rfc8960>.
+
+6.2.  Informative References
+
+   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
+              Label Switching Architecture", RFC 3031,
+              DOI 10.17487/RFC3031, January 2001,
+              <https://www.rfc-editor.org/info/rfc3031>.
+
+   [RFC6428]  Allan, D., Ed., Swallow, G., Ed., and J. Drake, Ed.,
+              "Proactive Connectivity Verification, Continuity Check,
+              and Remote Defect Indication for the MPLS Transport
+              Profile", RFC 6428, DOI 10.17487/RFC6428, November 2011,
+              <https://www.rfc-editor.org/info/rfc6428>.
+
+   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
+              and R. Wilton, "Network Management Datastore Architecture
+              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
+              <https://www.rfc-editor.org/info/rfc8342>.
+
+   [RFC8529]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
+              Liu, "YANG Data Model for Network Instances", RFC 8529,
+              DOI 10.17487/RFC8529, March 2019,
+              <https://www.rfc-editor.org/info/rfc8529>.
+
+   [RFC8530]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
+              Liu, "YANG Model for Logical Network Elements", RFC 8530,
+              DOI 10.17487/RFC8530, March 2019,
+              <https://www.rfc-editor.org/info/rfc8530>.
+
+   [RFC8532]  Kumar, D., Wang, Z., Wu, Q., Ed., Rahman, R., and S.
+              Raghavan, "Generic YANG Data Model for the Management of
+              Operations, Administration, and Maintenance (OAM)
+              Protocols That Use Connectionless Communications",
+              RFC 8532, DOI 10.17487/RFC8532, April 2019,
+              <https://www.rfc-editor.org/info/rfc8532>.
+
+   [W3C.REC-xml-20081126]
+              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
+              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
+              Edition)", World Wide Web Consortium Recommendation REC-
+              xml-20081126, November 2008,
+              <https://www.w3.org/TR/2008/REC-xml-20081126>.
+
+Appendix A.  Echo Function Configuration Example
+
+   As mentioned in Section 2.1.2, the mechanism to start and stop the
+   Echo function, as defined in [RFC5880] and discussed in [RFC5881], is
+   implementation specific.  In this appendix, we provide an example of
+   how the Echo function can be implemented via configuration.
+
+   module: example-bfd-echo
+     augment /rt:routing/rt:control-plane-protocols
+               /rt:control-plane-protocol/bfd:bfd/bfd-ip-sh:ip-sh
+               /bfd-ip-sh:sessions:
+       +--rw echo {bfd-types:echo-mode}?
+          +--rw desired-min-echo-tx-interval?    uint32
+          +--rw required-min-echo-rx-interval?   uint32
+
+A.1.  Example YANG Module for BFD Echo Function Configuration
+
+   This appendix provides an example YANG module for configuration of
+   the BFD Echo function.  It imports and augments "/routing/control-
+   plane-protocols/control-plane-protocol" from [RFC8349], and it
+   references [RFC5880].
+
+   module example-bfd-echo {
+     namespace "tag:example.com,2021:example-bfd-echo";
+     prefix example-bfd-echo;
+
+     import ietf-bfd-types {
+       prefix bfd-types;
+     }
+     import ietf-bfd {
+       prefix bfd;
+     }
+     import ietf-bfd-ip-sh {
+       prefix bfd-ip-sh;
+     }
+     import ietf-routing {
+       prefix rt;
+     }
+
+     organization
+       "IETF BFD Working Group";
+     contact
+       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
+        WG List:  <mailto:rtg-bfd@ietf.org>
+
+        Editor:   Reshad Rahman
+                  <mailto:reshad@yahoo.com>
+
+        Editor:   Lianshu Zheng
+                  <mailto:veronique_cheng@hotmail.com>
+
+        Editor:   Mahesh Jethanandani
+                  <mailto:mjethanandani@gmail.com>";
+     description
+       "This module contains an example YANG augmentation for
+        configuration of the BFD Echo function.
+
+        Copyright (c) 2021 IETF Trust and the persons identified as
+        authors of the code.  All rights reserved.
+
+        Redistribution and use in source and binary forms, with or
+        without modification, is permitted pursuant to, and subject to
+        the license terms contained in, the Simplified BSD License set
+        forth in Section 4.c of the IETF Trust's Legal Provisions
+        Relating to IETF Documents
+        (https://trustee.ietf.org/license-info).
+
+        This version of this YANG module is part of RFC 9127; see the
+        RFC itself for full legal notices.";
+
+     revision 2021-09-03 {
+       description
+         "Initial revision.";
+       reference
+         "RFC 9127: YANG Data Model for Bidirectional Forwarding
+          Detection (BFD)";
+     }
+
+     /*
+      * Groupings
+      */
+
+     grouping echo-cfg-parms {
+       description
+         "BFD grouping for Echo configuration parameters.";
+       leaf desired-min-echo-tx-interval {
+         type uint32;
+         units "microseconds";
+         default "0";
+         description
+           "This is the minimum interval that the local system would
+            like to use when transmitting BFD Echo packets.  If 0,
+            the Echo function as defined in BFD (RFC 5880) is
+            disabled.";
+       }
+       leaf required-min-echo-rx-interval {
+         type uint32;
+         units "microseconds";
+         default "0";
+         description
+           "This is the Required Min Echo RX Interval as defined in BFD
+            (RFC 5880).";
+       }
+     }
+
+     augment "/rt:routing/rt:control-plane-protocols/"
+           + "rt:control-plane-protocol/bfd:bfd/bfd-ip-sh:ip-sh/"
+           + "bfd-ip-sh:sessions" {
+       description
+         "Augmentation for the BFD Echo function.";
+       container echo {
+         if-feature "bfd-types:echo-mode";
+         description
+           "BFD Echo function container.";
+         uses echo-cfg-parms;
+       }
+     }
+   }
+
+Acknowledgments
+
+   We would like to thank Nobo Akiya and Jeff Haas for their
+   encouragement on this work.  We would also like to thank Tom Petch
+   for his comments on the document.  We would also like to thank Acee
+   Lindem for his guidance.  Thanks also to Jürgen Schönwälder, who was
+   instrumental in improving the YANG modules.
+
+Authors' Addresses
+
+   Reshad Rahman (editor)
+   Canada
+
+   Email: reshad@yahoo.com
+
+
+   Lianshu Zheng (editor)
+   Huawei Technologies
+   China
+
+   Email: veronique_cheng@hotmail.com
+
+
+   Mahesh Jethanandani (editor)
+   Xoriant Corporation
+   1248 Reamwood Ave
+   Sunnyvale, California 94089
+   United States of America
+
+   Email: mjethanandani@gmail.com
+
+
+   Santosh Pallagatti
+   VMware
+   India
+
+   Email: santosh.pallagatti@gmail.com
+
+
+   Greg Mirsky
+   Ericsson
+
+   Email: gregimirsky@gmail.com